The woman he marries must be a virgin.
13. And he shall take a wife in her virginity. More is required in the high priest, viz., that he should not marry a widow, nor a woman of any other tribe than his own. A question may indeed arise as to the latter clause, whether the plural word ought to be restricted to one tribe, Ainsworth’s literal translation is “a virgin of his peoples.”...
More was expected from a priest than from other people, but more from the high priest than from other priests, because upon his head the anointing oil was poured, and he was consecrated to put on the garments (Lev 21:10), both which were typical of the anointing and adorning of the Lord Jesus, with all the gifts and graces of the Holy Spirit, which he received without measure.
Commenting on Leviticus 21:10-15
And he shall take a wife in her virginity. One, and not two, or more, as Ben Gersom observes; and so Maimonides says (q), an high priest might never take two women together; for it is said, "a wife", or "woman", one, and not two; and so it is explained in the Talmud (r); for though polygamy was practised by the Israelites, and even by...