Welcome to Maseches Orlah. The mitzvah of orlah in the Torah means that fruit growing on a fruit tree during the first three years of its growth is forbidden to derive any benefit from - we do not eat it and we do not benefit from it at all. The fruit of the fourth year has the status of maaser sheni: it must be brought up to Yerushalayim and eaten there, or redeemed with money, and the money brought up to Yerushalayim.
The Mitzvah of Orlah Outside Eretz Yisrael:
The mitzvah of orlah applies not only in Eretz Yisrael but even outside the Land. This is a halachah l'Moshe miSinai, and although it is a mitzvah that depends on the Land, tied to the soil, it is nevertheless relevant outside Eretz Yisrael as well.
Counting the Years of Orlah:
Rosh Hashanah for orlah - the dividing line is the first of Tishrei, Rosh Hashanah.
When the count begins - once the tree has taken root, that is, forty-four days before Rosh Hashanah, that is counted as its first year of orlah, and the coming year is already considered the second year.
The fourth year - at the end of the third year the tree enters its fourth year, and even then the fruit that grows on it is not permitted immediately. Only fruit that blossomed in that fourth year after the fifteenth of Shevat is considered fruit of the fourth year, and it is neta revai, whose law is that of maaser sheni, as stated above.
Chapter 1, Mishnah 1:
The Mishnah here works with the principle that only a fruit tree planted for the sake of eating its fruit is subject to the prohibition of orlah; but if it was planted for another purpose, the mitzvah of orlah does not apply to it.
"Hanote'a lisyag ul'koros - patur min ha'orlah" - one who plants a fruit tree for the sake of a fence, or for the sake of beams and lumber, the laws of orlah do not apply to it.
Rabbi Yosi extends the principle even within a single tree:
"Afilu amar: hapnimi lema'achal v'hachitzon lisyag - hapnimi chayav v'hachitzon patur" - even when a person planted a row of trees for the sake of a fence and said that the inner side is intended for eating and the outer side for the fence, the inner part is obligated in the mitzvah of orlah and the outer part is exempt from it.
In summary: We opened by defining the mitzvah of orlah - the prohibition of benefiting from the fruit of the first three years, and the status of fourth year fruit as maaser sheni; we saw that the mitzvah applies even outside Eretz Yisrael, a halachah l'Moshe miSinai; we learned how the years of orlah are counted from the first of Tishrei, from the taking of root until the fifteenth of Shevat of the fourth year; and in Mishnah 1 we encountered the principle that only one who plants for eating is obligated in orlah, and Rabbi Yosi's novel teaching that obligation and exemption are determined even within a single tree according to the intent at the time of planting.