The field in a poker hand is the group of players who are still involved in a pot (that is, the set of players who have not yet folded).
The term is generally only used when speaking about general strategy (e.g. "bluffing is most effective against small fields."), and is rarely used when speaking about a hand in progress.
The phrase "thinning the field" is often used to describe a player who is raising in order to induce other players who are still in the pot to fold. Doing so reduces the size of the field, which is often described as "thinning" or "narrowing" the field.