The Torah commands special laws regarding the status of ritual purity and impurity when it comes to food and drink. To be defined as food, something has to be detached from the ground and potentially fit for human consumption. Accordingly, stalks of wheat in the field and food only fit for animal feed are not subject to ritual impurity.
Now, let’s say that something is detached from the ground and potentially fit to feed people. Even then, it will not contract impurity unless it has been rendered susceptible by becoming wet with one of seven liquids – water, dew, oil, wine, milk, blood or honey. Once wet with one of these liquids, the food remains susceptible to contract ritual impurity even when dry. Further, the Torah says “if water was placed on seeds…” from which the Sages determine that these laws only apply if the food item was intentionally made wet in a manner that satisfies the owner, for example, washing an apple.
The Sefer HaChinuch says that even though the Sages pretty much consider the reason for this mitzvah to be “because God said so,” he ventures to explain a part of it. He says that as a general rule, the special laws of an item do not apply until such time as that item is complete. For example, the kohein (priest) is to be given a portion of one’s dough. If one separates this portion from flour, it is ineffective. Here, too, the Chinuch hypothesizes, the law of impurity of food does not apply until such time as it reaches a condition that can be considered food. The stage where people wash something off prior to eating it is a pretty good indicator that it’s ready.
This mitzvah applies to men and women in all times and places in that food that meets the proper conditions can contract ritual impurity. However, in the absence of the Temple, it has no practical application, like most laws of purity and impurity. In the Mishna, different aspects of this mitzvah are the subjects of tractates Machshirin, Tohoros and Uktzin; in the Talmud, see Chulin 121a-b. This mitzvah is codified in the Mishneh Torah in the first chapter of Hilchos Tumas Ochlin. It is #98 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos; it is not counted by the Ramban (Nachmanides), nor is it listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.