Toharot means purity. Indeed this tractate has the same name is the entire Seder. There are three topics discussed in Tractate Toharot.
1) The purity of food and liquids. The Torah explicitly states that food and drink can be susceptible to impurity. For instance Leviticus 11:34 reads, "As to any food that may be eaten, it shall become unclean if it came into contact with water; as to any liquid that may be drunk, it shall become unclean if it was inside any vessel." The rabbis hold that the impurity of liquids is greater than that of food. This is expressed in several differences between how these materials convey impurity. Food conveys impurity to other food and liquids but it doesn't convey impurity to vessels. In contrast, impure liquids convey impurity to vessels as well as other food and liquid. Another stringency is that a liquid gets first degree impurity whether it came into contact with a "father of impurity" or a derivative thereof. If it now comes into contact with another liquid the second liquid also gets first degree impurity. This continues on no matter how many subsequent liquids are touched (see Parah 8:5-7). The third stringency is that the smallest amount of liquid conveys impurity, whereas food must be the size of an egg in order to defile something else.
2) Doubtful impurity in the private and public domain. The tractate deals with the category of "doubtful impurity" meaning cases where something may or may not have been impure in either the public or private domain. From the laws concerning the Sotah (the doubtfully adulterous woman) the sages derived that all cases of doubtful impurity arising in the private domain are deemed as impure. The Sotah woman is prohibited to her husband because she may have been defiled by adultery. This doubtful impurity occurred in the private domain (where she may have hid from her paramour). Just as the doubtful impurity that occurs with the Sotah happens in the private domain, so too all cases of doubtful impurity that occur in the private domain are deemed impure.
3) The impurity of the am haaretz. In the Mishnah, an am haaretz is a person who is not scrupulous about the observance of certain laws, including the purity laws and the tithing laws (see Tractate Demai). The rabbis decreed that he conveys impurity as does a zav, but they limited this to things with which he has contact. He does not convey impurity by carrying or by shifting (without touching).