Any five-letter word on the game’s word list. A guess that is too short or not on the list is rejected, with a message, and does not use up a guess.
A guess must be exactly five letters, a to z, and be on the game’s word list. If it is too short you see “Not enough letters”, and if it is not on the list you see “Not in the word list”. Neither one uses up a guess.
The list is larger than the set of possible answers, so you can use unusual words to test letters. The answers themselves are everyday words (how many guesses you get). See also the Daily Word tips.