Priming is where if you're introduced to an idea, you'll more readily identify related ideas.
Let's take an experiment as an example, again from Less Wrong:
Suppose you ask subjects to press one button if a string of letters forms a word, and another button if the string does not form a word. (E.g., "banack" vs. "banner".) Then you show them the string "water". Later, they will more quickly identify the string "drink" as a word. This is known as "cognitive priming" ...
Priming also reveals the massive parallelism of spreading activation: if seeing "water" activates the word "drink", it probably also activates "river", or "cup", or "splash."