Welcome to Maseches Moed Katan. The tractate carries this name because it deals with the laws of Chol HaMoed - 'moed katan', a minor festival, and that is the essence of Chol HaMoed: it is part of Yom Tov, yet not all of the laws of Yom Tov apply to it. There are activities that are forbidden on it and activities that are permitted, and that is the subject of the tractate. Toward its end the tractate will also enter into the laws of mourning, and this is because there is a connection between the laws of mourning and the laws of Chol HaMoed.
Mishnah 1 - Irrigating Fields on Chol HaMoed and in Shevi'is:
The Mishnah opens with a discussion of irrigation and other agricultural needs on Chol HaMoed, and its words are: "Mashkin beis hashelachin bamoed uvashevi'is" - one may irrigate a beis hashelachin both on the moed, meaning Chol HaMoed, and in shevi'is, the Shemittah year. To understand the terms:
Beis hashelachin - a field that requires irrigation, which cannot survive on rainwater alone.
Beis habaal - a field that can survive on rainwater alone.
The reason for the leniency:
On Chol HaMoed - the halachah is that labor which is a 'davar ha'aved', meaning that failing to do it will cause a monetary loss, is permitted on Chol HaMoed.
In shevi'is - although one may not work the field during Shemittah, irrigating a field is not a Torah violation but only a rabbinic one, and where a loss is liable to result, and even where the field itself would be ruined for the future, the Sages permitted doing so.
It is worth noting that there are opinions among the Rishonim that in Shemittah one may irrigate even a beis habaal, a field that survives on rainwater, and the reason the Mishnah limited the leniency to a beis hashelachin alone is because Chol HaMoed is included here as well, and on Chol HaMoed the leniency is limited to a beis hashelachin. Others understand, by contrast, that even in Shemittah the matter is limited to a beis hashelachin alone.
From where one may draw the water:
The Mishnah continues and defines the water source for irrigating these fields: "Bein bemayim sheyotz'in batechilah, bein bemayim shelo yotz'in batechilah" - whether it is a spring that has only now begun to flow, or an established spring that has already existed for some time. The novelty in permitting the new spring is that new springs tend to erode immediately and their walls tend to collapse inward, and a person may come to repair them on Chol HaMoed - a repair which in itself is forbidden on Chol HaMoed.
What is forbidden because of excessive exertion:
The underlying principle is that minor exertion was permitted on Chol HaMoed, but not great exertion. And from here:
Standing rainwater - one may not irrigate from it, since carrying the water in buckets from the place where it stands involves great exertion. A spring can be diverted, and the one irrigating need not carry out the transfer himself, which is not the case with standing water.
A deep well - a very deep well, from which drawing water requires great effort, was likewise not permitted for the purpose of irrigating a beis hashelachin.
Ugiyos for vines - one may not make the trenches around the base and roots of the vines. These trenches are meant to fill with water, and they too involve great exertion, which was not permitted on Chol HaMoed.
In summary: In this Mishnah we learned that one may irrigate a beis hashelachin - a field that requires irrigation - both on Chol HaMoed, by virtue of the rule of davar ha'aved, and in shevi'is, where the prohibition of irrigating is only rabbinic and was permitted where a loss is involved. The leniency applies both to a new spring and to an established spring, but not to standing water, not to a deep well, and not to making ugiyos for vines - for all of these involve excessive exertion, which was not permitted on Chol HaMoed.