Meat fills about half of the Protagonist's health and one fourth of his Stamina. It is recommended that meat is not consumed and is just given to Victim 18 because in order to get the full 120% completion the player must not eat meat. It is also Human flesh, so only cannibals eat it and it is considered inhumane to consume.
Meat be found in the Line 4, where two Cannibals will freely give the player a slice of Meat. Another slice of meat can be found in the makeshift shelter just south of Henry's apartment, conveniently left of the pole where you climb up before finding the strange mutant and shotgun.