The writer speaks of two large divisions of offerings which were given in the temple. Gifts and Sacrifices. They are closely related, but distinct. They are visible in the church today by the divisions of tithes and offerings. Tithes are the required first 10% of all your increase… offerings are the voluntary gifts in excess of the basic tithe. No offering CAN be given until the tithe is satisfied, for the tithe is NOT ours but God's, and to fail to give it is to rob God … you cannot give a gift out of something that is stolen, especially when the one receiving the "gift" is the one from whom it was stolen in the first place. In the same manner, the Old Testament picture that illustrates this principle is where the writer is pressing the Hebrews to think. God is OWED their hearts, and no mere physical gift can even be acknowledged until THAT debt is paid!