Music made by former Hair and Glam Metal bands. This refers to all the albums they make after the 80's when they stop playing the traditional music they used to play and play more modern style music. The joke refers to the fact that the hair metal bands don't have hair anymore because they are old making their new material referred to as bald metal.
Dokken used to make awesome Hair Metal now they make terrible Bald Metal.
Winger's title album was a great hair metal album (1988) but the album Winger IV (2006) is a crappy bald metal album.