The term ḥerem is found in a number of Semitic languages, meaning a thing that is sanctified to God. In the Torah we find it relating to a number of different situations, meaning something that must be utterly destroyed, something that is forbidden to all, or killing. Another meaning of the word is a very serious oath, one whose abrogation would mean death. In Rabbinic literature there are two additional ways in which the word is used – as a serious type of excommunication or a severe prohibition established by the community or by the Rabbinic court.
In our Mishna, the word ḥerem is used to mean that the person has sanctified his property by means of that expression. In contrast with ordinary sanctity, which can be redeemed, a ḥerem is totally forbidden and removed from the possession of the original owner. Thus we find that someone who declares his ancestral plot of land to be sanctified in an ordinary way is encouraged to redeem it before the Jubilee year. If, however, the method of sanctification was by means of ḥerem, the land immediately is transferred to the kohanim and he no longer has any rights to it.
For this very reason, the Mishna explains that a person can only declare his own ancestral field ḥerem. If he has purchased a field from another that is supposed to be returned to the original owner in the Jubilee year, then the purchaser cannot declare it to be ḥerem since it does not truly belong to him and a person does not have the ability to declare something that does not belong to him to be ḥerem.