What you have learned about an opponent's tendencies by watching (player reads).
From how often they enter, how often they raise, whether they fold to bets, and the hands they show at showdown (when the remaining players reveal their hands), you decide the opponent's type. A read is often a guess about one hand; reads usually means a player's overall tendencies. Example. Out of 20 hands they played 9 (45%) with few raises, so you read them as a Station. Once you know the type, you know which way to adjust, such as fewer bluffs and more value bets.