siezen means “address someone formally”

More examples

Viele Kollegen siezen ihren Vorgesetzten aus Höflichkeit.

Many colleagues address their supervisor formally out of politeness.

Two speakers maintain a formal conversational boundary by using Sie instead of du.

duzen: Using the familiar du form when addressing someone.
anreden: Using a title or honorific to address someone.

jemanden mit dem förmlichen „Sie“ anreden

Siezen developed by turning the pronoun Sie into a verb with the suffix -zen, a formation also seen in duzen.

The verb siezen is written in lowercase, while the formal pronoun Sie remains capitalized.