A “mic drop” is figuratively a way of saying, symbolically, that what someone said was so utterly amazing all they need to do is exit the stage. It’s used quite liberally in pop culture nowadays, but it started out as a joke. You can thank Eddie Murphy (after a particularly awful performance, a character played by Murphy, in the movie titled “Coming to America”, drops the mic and walks off the stage. It was meant to be hilarious, and it was, but it evolved into so much more).
For the purposes of this post, the “mic drop” came in the form of a statement made by Georgia running back Sony Michel.