Tractate Orlah is concerned with fruit that grows on a tree during its first three years of growth. The laws are all based on Leviticus 19:23-24:
When you enter the land and plant any tree for food, you shall regard its fruit as forbidden (arel). Three years its fruit shall be forbidden (arel) for you, not to be eaten. In the fourth year all its fruit shall be holy, (set aside) for jubilation before the Lord; and only in the fifth year may you use its fruit—that its yield to you may be increased. I the Lord am your God.
According to this verse, fruit which grows on a tree during its first three years is prohibited. The rabbis add that it is prohibited not just to eat this fruit, but also to derive any benefit from it.
The years of a tree are counted not by how long ago the tree was actually planted but by the calendar years. The first of Tishrei is the date that is considered the first of the year for orlah (see Rosh Hashanah 1:1) so a tree planted before the first of Tishrei (actually thirty days before the first of Tishrei so that it has time to grow roots), is considered to already be in its second year.
In its fourth year the fruit is called “fourth year produce” or “neta revai.” The mishnah dealt with this subject at the end of tractate Maaser Sheni because this fruit is treated similarly to second tithe.
Our tractate also deals extensively with the subject of orlah fruit that has become mixed up with other permitted produce. It also deals with mixed seeds and terumah that have become mixed up with other produce, because these halakhot are all similar.