The word "Ohalot" means "tents" and it refers to the halakhah found in Numbers 19 (quoted below) according to which a person or vessel found in a tent with a dead body is impure even without direct contact with the body. In other words, the tent causes the defilement of the corpse to spread throughout the tent and defile everything in the tent. The rabbis define "tent" in a very broad manner. Basically, any covering that hangs over a corpse (or piece thereof) can convey the impurity to anything else which it overhangs. Basically, the only requirement is that this "overhanging" or "tent" be at least one handbreadth long, one handbreadth wide and one handbreadth in height. And just as a "tent" (I will try to use the term "ohel" as well) conveys the impurity to that which is found beneath it, it also prevents the impurity from spreading to that which is above the tent. In other words it conveys and confines at the same time. If a corpse is above the ohel, that which is below the ohel is not defiled.
Tractate Ohalot also deals with other issues concerning the impurity of a corpse. The Torah specifically states that anyone that has contact with a corpse is impure for seven days. The rabbis add that carrying a corpse also defiles, even without contact. The Torah also states that a person who had contact with a dead body defiles others and vessels, but only for a period of one day. Thus the dead body itself is the "father of the fathers of impurity" and the person (or vessel) who had contact with a dead body is a "father of impurity."
Tractates Ohalot and Negaim, the next tractate, were already famous in the Talmudic period for their difficulty. In a passage which I happened to have just taught a few days ago (Bavli Pesahim 50a) R. Yohanan says that these two tractates are difficult in this world, but will be easier in the world to come. In Bavli Hagigah 14a, Rabbi Elazar ben Azarya scolds Rabbi Akiva for dealing in matters of aggadah and tells him to go study Ohalot and Negaim. He chooses these two tractates due to their depth and difficulty. We are a bit luckier than Rabbi Akiva for we have many commentaries on which to base our study, including a relatively recent modern commentary by Professor Abraham Goldberg (whose son-in-law is a colleague of mine at the Conservative Yeshiva). These laws will be difficult, but perhaps by making them a bit easier than they were at first, we too can have a small taste of the world to come. Below are the verses most relevant to our tractate. Good luck!