Deuteronomy 12:15 "Nonetheless, you may slaughter animals and eat their meat to your heart's desire"

An animal that was designated to be sacrificed to HaShem is considered special and holy.

It must not have any type of blemish, since a blemish makes it unfit for sacrifice. Should it develop a blemish, it cannot be offered as a sacrifice.

Since the blemished animal was considered special, because it was originally designated as a sacrifice, it must be redeemed. (See Negative Mitzvot 92-97.)

The Torah commands us to choose a different animal for the sacrifice in place of the first. This is called "redeeming" the first animal. Afterwards, the first one can be eaten like any other animal.