What does Exodus 28:43 mean?

Aaron and his sons must wear these clothes whenever they enter the tent of meeting or approach the altar to serve in the holy place, so they don't bring guilt on themselves and die. This is a permanent rule for Aaron and his descendants. Exodus 28:43 - Modern Text Bible

(These instructions are serious—following them is a matter of life and death, and they're meant to be followed by every generation of priests.)