To tithe means to give 10% (a tenth part).
"The dedication of a tenth to God was recognized as a duty
before the time of Moses. Abraham paid tithes to Melchizedek
(Gen. 14:20; Heb. 7:6); and Jacob vowed unto the Lord and
said, "Of all that thou shalt give me I will surely give the tenth
unto thee." The first Mosaic law on this subject is recorded
in Lev. 27:30-32. Subsequent legislation regulated the destination
of the tithes (Num. 18:21-24, 26-28; Deut. 12:5, 6, 11, 17; 14:22,
23). The paying of the tithes was an important part of the Jewish
religious worship. In the days of Hezekiah one of the first results
of the reformation of religion was the eagerness with which the
people brought in their tithes (2 Chr. 31:5, 6). The neglect of
this duty was sternly rebuked by the prophets (Amos 4:4; Mal. 3:8-10)"
(Smith's Bible Dictionary).
God changed the law after Christ's death (Heb. 7:12-14). We live
under the law of Christ
Christians do not tithe (i.e., give 10%). Christians are commanded
to give on the first day of the week as they have purposed in their
hearts. See Financing
the Work of the Church.