Before us is Mishnah 2 in the first chapter of Maseches Rosh Hashanah. In the previous Mishnah we learned that there are four new years, and our Mishnah likewise presents a list of four: four times in the year when the world is judged, each time for a different matter.
The four times of judgment:
On Pesach - for the grain: it is decreed how much grain there will be in the world.
On Atzeres (Shavuos) - for the fruit of the tree: their fate is determined.
On Rosh Hashanah, the first of Tishrei - "kivnei maron": all who come into the world pass before the Holy One, Blessed is He, like sheep passing one after another, as it says: "hayotzer yachad libam, hameivin el kol ma'aseihem" - He fashions their hearts together, He understands all their deeds: He is the One who judges them all.
On Sukkos - for the water: the world is judged regarding the rains.
The offerings that express the judgment of the three festivals:
On these three festivals - Pesach, Shavuos and Sukkos - we bring offerings that represent the judgment of that particular time:
On Pesach - the Omer offering: it comes from barley, and it expresses the fact that we are about to be judged regarding the grain.
On Shavuos - the two loaves: they come from the new grain and are made from wheat. According to one opinion, the Tree of Knowledge was wheat, and for that reason wheat has a connection to being a fruit of a certain kind, and corresponding to this we are judged on the fruit of the tree.
On Sukkos - the water libation: a special mitzvah to pour water upon the Altar in the Beis HaMikdash, since at this time we are judged regarding the water.
In summary: in this Mishnah we learned the four times of judgment - on Pesach for the grain, on Atzeres for the fruit of the tree, on Rosh Hashanah for all who come into the world as they pass before Him like sheep, and on Sukkos for the water - and the offerings of the festivals, each one expressing the judgment of its time: the Omer, the two loaves and the water libation.