What’s over shaving?
Going over the same area repeatedly. Most people do it to some degree I expect. Over do it and you can be in trouble. A very good lather with good residual slickness makes it surprisingly doable with solid technique. Chasing BBS or excessive clean-up would also qualify in my book.
What’s brush burn?
As Thom says. Face lathering with a too dry or underloaded brush, for too long and too aggressively will do it for me.