The upper strut plate, or mount, has a small bearing in it so the strut can pivot in the direction of steering. These can wear out so it's not uncommon to have them replaced when new struts are installed.
It is not something you can replace yourself in your driveway. It has to be done by a professional.
My suggestion is to have both front mounts replaced along with new struts. Your mechanic will have to take the struts apart anyway.
Good luck!