Albert Barnes Commentary Numbers 20:26

"and strip Aaron of his garments, and put them upon Eleazar his son: and Aaron shall be gathered [unto his people], and shall die there." — Numbers 20:26 (ASV)

The priestly garments, with which Moses had invested Aaron (Leviticus 8:7–9), were put upon Eleazar as a solemn transfer of Aaron’s office to him; compare (1 Kings 19:19).