Introduction to Maasrot
First tithe, the first tenth of ones produce, are given to the Levites. The only verses in the Torah which deal with this tithe is Numbers 18:21-24:
21 And to the Levites I hereby give all the tithes in Israel as their share in return for the services that they perform, the services of the Tent of Meeting. 22 Henceforth, Israelites shall not trespass on the Tent of Meeting, and thus incur guilt and die: 23 Only Levites shall perform the services of the Tent of Meeting; others would incur guilt. It is the law for all time throughout the ages. But they shall have no territorial share among the Israelites; 24 For it is the tithes set aside by the Israelites as a gift to the LORD that I give to the Levites as their share. Therefore I have said concerning them: They shall have no territorial share among the Israelites.
The topic that is discussed throughout most of the tractate is when does produce become liable for tithes? To put this question another wayup until what point can one eat produce without tithing it? There are several potential occurrences that make produce liable for tithing, such as making it into a pile, taking it into ones courtyard or house or cooking it. Before any of these actions happen, one can eat produce in a non-fixed meal, as a snack, but not as part of a more formal meal. For instance, if one is in the field, one can pick grapes directly off of the vine without tithing them. However, once one brings them into ones house in order to drink the wine with a meal, he cannot drink it until it has been tithed.
The word maasrot literally means tithes, but it often refers to terumah as well as tithes. The reason is that tithes are terumah would have been typically separated from produce at the same time. So many of these mishnayot deal both with terumah and tithes.
The mishnayot throughout the tractate assume that tithes are given to the Levite. However, there is ample evidence that in the Second Temple period, tithes were frequently given to priests, either along with Levites, or instead of Levites. Since this issue is not really relevant to our tractate, I am not going to delve further into it.
We should also note that unlike terumah, which is strictly prohibited to non-priests, anybody can eat tithes. They are not called holy by the Torah. Therefore, there is no discussion about tithes that get mixed up with hullin, a discussion which took up a large portion of Terumot.
Good luck in learning the Tractate!