What does Matthew 27:7 mean?

Religious leaders who had paid Judas for betrayal (Matthew 26:14–16) still saw his actions as despicable and immoral. When Judas attempted to return the money, they refused it (Matthew 27:3–6). Not only that, but they also saw the money as tainted by Judas' actions and unsuitable to put in with other temple funds. Most likely, they don't want to leave more evidence of their illegal plot (John 11:48–53). So, they put their heads together and come up with a creative legal solution.

Apparently, interpretations of laws such as Deuteronomy 23:18 only meant the coins could not be "brought into" the temple. They did allow for such money to be used outside of Judaism for the common good of the community. The chief priests use Judas' returned fee to buy a field to be used as a burial place for foreigners in Israel. It's not known why it is called the "potter's field." Scholars speculate that it may have once belonged to a potter or it is possible that potters took clay from the field for their pots.