An asham talui (“conditional guilt offering”) was brought under certain circumstances when a person thought he might have committed a sin but wasn’t sure. The situation is as follows:
Let’s say that there’s an act which, if committed intentionally, carries the penalty of kareis (spiritual excision) and, if committed unintentionally, requires a korban chatas (sin offering). Now let’s say that a person isn’t even sure he did such an act, though he knows he did some act. For example, there’s a piece of cheilev (one of the forbidden fats) and a piece of shumen (regular kosher fat) lying on a table and he ate one. The other piece got lost with the result that there’s no way of knowing whether he ate something permissible or something seriously impermissible. (We’ll come to the prohibition against eating cheilev in Mitzvah #147.) In such a case, the person would bring an asham talui, for uncertain guilt.
An asham talui is only required in a case where a forbidden object was definitely present, as in our example where the person doesn’t know which piece he ate but one was definitely cheilev. If there’s a single piece of fat and he simply doesn’t know its nature, then a forbidden object is not definitely present. Certainly, eating such a piece is ill-advised, but one would not bring an asham talui in such a case.
This sacrifice only works conditionally. If a person brings an asham talui, but later it is discovered that he did in fact commit the sin, he must then bring a regular sin offering.
The basis of this mitzvah is to condition people to tread carefully in their deeds and be aware of what it is they’re doing. Maybe, before you pop that piece of food in your mouth, you want to check if it’s actually kosher? A little effort now saves a lot of trouble down the line.
This obligation applies to both men and women when the Temple is standing. It is discussed in the fourth chapter of the Talmudic tractate of Kerisos and is codified in the Mishneh Torah in the eighth chapter of Hilchos Shegagos. This mitzvah is #70 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.