Aftermath, in rap, primarily refers to the legendary record label founded by Dr. Dre. Aftermath would later be the home of several hip-hop icons like Kendrick Lamar, Eminem, and others. Others might use like:
You should be scouted by Aftermath. Your rapping skills are slick!
Some might use it as a slang that indicates some measure of success, especially in music. You may hear someone using it like:
I’m in Aftermath. I am a rap superstar!
It is still popular up to this day because of the reputation that Aftermath has.