Flood is an informal game term referring to the Free Peoples player filling the Twilight Pool with many tokens, usually by playing many high-cost cards. Flooding can be a strategy: if you've played many expensive cards, that generally means you've built a strong Fellowship. Often, however, it is by mistake. A common example of routine flooding would be using Denethor, Lord of Minas Tirith (10R28) to tutor a couple Gondor cards, especially companions, then moving to a site with 15 or more tokens in the pool.
Flooding is generally dangerous, as your opponent has great freedom to then play Shadow cards, particularly minions. Flooding against Swarm or Beatdown can have devastating effects, as the Shadow player can either play a great deal of their cheap minions or play a few powerful minions. However, it can also be a strategy: once your opponent has all of the Twilight they need to play all of their Shadow cards, further Twilight is generally harmless. Plus, some cards, particularly Gandalf culture cards like Gandalf, The White Wizard (4C90) and Deep in Thought (3C30), benefit from a full Twilight pool. Some decks, such as Ents, often plan to flood the pool one turn, weather the storm, and move down the path with a strong Fellowship, only slightly worse for wear. Even this is not without risk: many minions, like Úlairë Attëa, Keeper of Dol Guldur (1R229), can make use of excess Twilight, and cards like Greed (1R125) or Rapid Reload (15U89) can turn Twilight directly into pain.
One way to mitigate the risk of flooding is managing your opponent's hand. This can be with cards like Sting (1R313) to see what cards they have in hand, or cards like Far-seeing Eyes (1C43) to force them to discard cards from hand. Some cards, like Curse Their Foul Feet! (1R36), do both.
Flood is the opposite of choke.