John Gill 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)

And strip Aaron of his garments
His priestly garments, as the Targum of Jonathan, and so Jarchi:

and put them upon Eleazar his son ;
thereby declaring him to be high priest in his father's stead:

and Aaron shall be gathered unto his people, and shall die there ;
the phrase of gathering to his people is here explained of his dying.