The Literal Commandment
He shall make restitution for that which he has sinned against the holy thing, and shall add to it a fifth part of it and give it to the priest. The priest shall then make atonement for him with the ram of the guilt offering, and it will be forgiven him. (Leviticus 5:16)
The "holy thing" in this verse is the food that is tithed to G-d or the food that is offered as part of a sacrifice to G-d: it is considered a breach of holiness to benefit from these things. If a person eats something that is devoted to G-d then he is to repay what was eaten plus one-fifth (20%) and bring an offering of a ram for his guilt.
For more details about the sacrifices we highly recommend the article from Psalm11918.org regarding
What Scripture Says About the Sacrifices. It provides details about the guilt offering: what the offering was and when it was used. They also have a separate article regarding the purpose of the sacrifices.