Leviticus 27:25

And all thy estimations shall be according to the shekel of the sanctuary: twenty gerahs shall be the shekel.

The sanctuary named here is the Lord's tabernacle, God's dwelling place. The sanctuary shekel is set as the standard measure for the valuations laid out in this chapter and for valuations in Israel generally. That standard shekel is defined as twenty gerahs.

The verse stands within the law by which a person or member of a household devoted by a special vow could be redeemed according to the priest's valuation. That valuation was to be reckoned by the sanctuary shekel. The wider chapter to which it belongs is a record of laws concerning redemption showing how the law of tithing was carried out in Israel.

Tithing itself was no new requirement at that time, for it was already known to Abraham and Jacob. Under the law of vows, Jephthah's daughter could not lawfully have been offered as a burnt offering. Because she was not redeemed, his vow devoted her instead to the Lord's service in perpetual virginity rather than to sacrifice.