Wax seals are an old-fashioned way to seal envelopes and documents so it’s obvious when they’ve been tampered with. You melt wax over paper, then use a small stamp to press a symbol into it. Only easy way to read the thing is to break the wax. When the symbol belongs to somebody powerful, like a king (or an archmage!), and you get correspondence from them with the seal intact, you know you’re getting an important message for your eyes only.
These days you mostly see wax seals on fancy wedding invitations — but in a fantasy setting, you have more options!
Melt some wax over the edge of a closed door, stamp it with the archmage’s seal, and prepare your Cosmic Binding spell, and the next person who opens the door is frozen in time and space.
Melt wax over the edge of a treasure chest lid, store your Miasma spell in it, and whoever tries to steal your stuff gets a face full of foul smoke.
Melt wax over your wedding invitation, cast your Charm spell on it, and people will want to come to your wedding! (Until the spell wears off.)
Hope this helps!