Not just a well-known song from the band, No Doubt, the expression hella good became popular slang in the Millennium, too. The word hella means "to the extreme" and when put in front of the word good, it translates to something that is "extremely good."
Not to be confused with hella cool, which was a 90s staple. Instead, hella cool morphed into hella good in the 2000s, like any good little catch phrase that wants to stay relevant should do.
And, hella still remains a Bay Area vocabulary staple (so, don't be shocked if your friend from San Francisco continues to throw this word your way).