Used to describe something or someone as being superior, or excellent.
So superior that it is incapable of being changed from its level of 'greatness', hence "solid"
Like a solid foundation, what is being described as "solid" is so excellent that it cannot be made less awesome.
"Son, those new two-tone kicks are indeed 'solid'!"
"Hey Joe, that new guitar riff sure is 'solid'"
"Check out Captain Cookout's new album! Its 'SOLID'!"