Me’ilah (“misappropriation”) refers to making personal use of items sanctified for Temple use. The Torah tells us that if one misuses such holy items, he must do three things: (1) He has to pay back the value of what he took; (2) he must add an additional 20% as a penalty (maybe not; see below); and (3) he must bring a guilt offering. The repayment and the fine are given to the kohein (priest) for the Temple treasury, after which the kohein offers the sacrifices to atone for the misappropriation.
This law is true for any misuse of sanctified items, regardless of their nature. Even if a non-kohein merely eats a kohein’s terumah, he must repay it with the penalty added and bring the guilt offering.
The reason for this mitzvah is to instill in every person an appropriate awe of the Temple. If you were visiting the White House and passed by a buffet table, you probably wouldn’t just take something and pop it in your mouth. (If you did, the Secret Service would probably make their displeasure known in due course.) Even more so, one shouldn’t visit the Temple and just help himself to whatever’s laying around. “Oh, this meat looks good!” “Oooh… soothing oil!” etc.
The Talmud in Me’ilah (19b) tells us that a person cannot misappropriate an item after another person has already done so. The exceptions are riding on an animal designated for a sacrifice or drinking from a Temple cup, as these items do not depreciate in value from the first misuse.
Actually, the 20% penalty was 25% of the misappropriated property. The Talmud in Baba Metzia (54a) explains that it is an “exterior fifth” (that it, 20% of the principal and the penalty combined) rather than an “interior fifth” (20% of the principal alone). So, if one misused an object worth $100, he would repay $125. The penalty ($25) is 25% of the principal, but it is 20% of the total amount being repaid.
The obligation to properly restore misused Temple items applies to both men and women when the Temple is standing. It is the subject of the Talmudic tractate of Me’ilah and is codified in the Mishneh Torah in Hilchos Me’ilah. This mitzvah is #118 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.