A speed bump (commonly written as speedbump, with no space) is a disparaging name for a Free Peoples character that its owner intends to sacrifice in a skirmish, ideally after first loading up with wounds to the point of exhaustion. They represent a small inconvenience that almost any minion will easily overcome and often overwhelm, but in doing so be "slowed down" and have less opportunity to cause any lasting harm. Sometimes this is done out of necessity. When the Free Peoples player is backed into a corner and has to choose a companion to be killed, having already planned on a particular character's death makes the decision easy. Other times this is done for the sake of convenience. Players might choose to kill off such a character to preserve vitality on more important companions, save skirmish events and abilities for later, or even just reduce the number of companions in the fellowship and add less twilight per move or stay below a threshold like the Rule of 6.
Of course, a Free Peoples character can be killed without being "a speed bump." Calling a character a speed bump implies a certain contempt and triviality. These are always characters of minimal importance to the Free Peoples player and, by extension, all Shadow players. The only accomplishment an opponent can claim in killing a speed bump is counting down against the Rule of 9, unless that character was an ally in which case there is no sense of accomplishment at all. Pippin, Hobbit of Some Intelligence (1R307) and Thrarin, Dwarven Smith (1U27) are among the most obvious and popular examples, often called "Pippin, Hobbit Speedbump" and "Thrarin, Dwarven Speedbump" in recognition of this.
The question then becomes, why put an unimportant character in a deck at all? First, unimportant is not the same as useless. Speed bumps typically provide some value in their game text even if they aren't essential to the deck at large. Second, the dead pile is a resource just like twilight and companion vitality. If one copy of a character destined for the dead pile does the job of two or three cards needed to keep an important companion alive, that can be a more efficient use of resources and might even free up some deck space. Third, remember that a Fellowship only needs to make 8 moves. Stalling an opponent with meaningless victories while your Ring-bearer coasts to the end is as valid a strategy as any other. Finally, they provide a good contingency. When things go south and your best companions die unexpectedly, what was intended to be a speed bump can suddenly become crucial to your survival.
Because the importance of a character is subjective, what is a speed bump in one deck - or even one particular game - can be a core companion in another. Legolas, Greenleaf (1R50) is sometimes be splashed to kill one or two minions in archery and die to another (not necessarily all in the same turn) without any investment from the Free Peoples player. In other decks he is prioritized for healing and relied upon for many turns, killed only as a last resort. Sam, Son of Hamfast (1C311) is commonly exhausted immediately after he is played to remove the starting bid, then kept on the sidelines until a troublesome minion like The Balrog, Durin's Bane (2C51) needs a skirmishing partner. Against a corruption deck, however, the Free Peoples player may instead want to keep Sam alive until site 6 to heal him and remove additional burdens or even make Sam the Ring-bearer.
When a deck's helpful characters are intentionally killed in skirmishes they aren't usually considered speed bumps, even when the sacrifice isn't strictly necessary (i.e., the Free Peoples player could have assigned minions in a way that no companions die). However, in those circumstances they are sometimes said to be "used as a speed bump" or simply "speed bumped." They are fulfilling the "role" of a speed bump without actually being one since the loss of that character will have a lasting impact on how the Fellowship copes for the rest of the game. Occasionally players will use this sort of usage to poke fun at important characters that are killed too, especially when strong characters are overwhelmed thanks to Shadow skirmish pumps or important characters are inadvertently killed by Free Peoples error. For example, one might say "Gandalf was just a speed bump to The Balrog" after the Free Peoples player carelessly assigned a 7-strength Gandalf to the 17-strength Balrog without realizing it would overwhelm him.
If the Free Peoples player is obliged to kill a specific character as soon as possible, that character is likewise not really "a speed bump" unless they were already considered one. A companion bearing Sense of Obligation (12U170) may find themselves hopelessly assigned to the strongest Nazgul on the table, but that's the Shadow player making a meaningful impact with their cards rather than the Free Peoples player depriving them of a meaningful impact. Still, as with circumstances where other helpful characters are sent to their doom by the Free Peoples, these companions are could be compared to speed bumps as a reflection of how useless or downright detrimental they have become. The only real contribution they have left for the Fellowship is by dying the most useful death possible.
The concept of an unimportant character who is expected to die is not unique to this game and there are many ways to describe the same basic phenomena. "Red shirt" from Star Trek most closely captures the trivial nature of speed bumps, while other popular terms such as "meat shield" and Magic: the Gathering's "chump blocker" could describe any character who dies for the sake of more important characters.