A pump is a card played to temporarily "pump up" (increase) a character's strength. Generally, pumps are played during the Skirmish Phase, after assignments have been made.
Most pumps are events like Flurry of Blows (2C5) . Event pumps usually add between +2 and +4 strength, sometimes with another bonus. (The most common additional bonuses are Damage +1 or healing.) Pump events tend to add less strength when they have relatively few conditions, and more strength when they have more conditions. These sorts of conditions include more spotting requirements, affecting fewer different characters, exerting a character, discarding cards, paying more Twilight, or skirmishing a certain type of character. Flurry of Blows is part of a cycle of cards in Fellowship of the Ring that add +2 under broad conditions and +4 under strict conditions, and that cycle sets the curve for all later sets. Pump events are generally less popular for Shadow deck than Free Peoples, because the Shadow side is limited in the Twilight they can spend, and every pump could instead have been another minion to play.
Some characters, like Gimli, Son of Gloin (1R13) pump themselves, and some other card types, like Saruman's Ambition (1C133) , can be discarded to pump characters. Since most pumps are Skirmish: special abilities, they end at the end of that particular skirmish unless the card specifies otherwise (such as Savagery to Match Their Numbers (1R139) ).
Cards that grant a continuous bonus, like Hobbit Sword (1C299) , are generally not considered pumps. Cards like Enraged (6R26) that grant a temporary strength bonus that starts before the Assignment Phase could theoretically be called pumps, but they're so rare that they're not often referred to at all.