“Temurah” means “substitute” and Tractate Temurah deals with cases where a person tries to substitute a non-sacred animal for another animal that has already been dedicated to be a sacrifice. This subject is dealt with in two parts of Leviticus 27, which I have quoted below:
Leviticus 27: 9-10 9 If [the vow concerns] any animal that may be brought as an offering to the Lord, any such that may be given to the Lord shall be holy. 10 One may not exchange or substitute another for it, either good for bad, or bad for good; if one does substitute one animal for another, the thing vowed and its substitute shall both be holy.
Leviticus 27:32-33 32 All tithes of the herd or flock — of all that passes under the shepherd's staff, every tenth one — shall be holy to the Lord. 33 He must not look out for good as against bad, or make substitution for it. If he does make substitution for it, then it and its substitute shall both be holy: it cannot be redeemed.
Since there are not that many laws that are connected to substituting non-sacred animals for sacrifices, our tractate also deals with the status of the offspring of dedicated animals. The commonality between these two subjects is that the sanctity of one animal (the substitute or the offspring) comes as a result of the sanctity of the other animal (the original sacrifice or the parent).