What the actual fuck is brainrot terms?

I was playing an online game and some random kid started spewing shit like "Lil bro, based on your outfit, you have 0 rizz." Excuse me but what the actual fuck is Rizz and lil bro? Bro was speaking that even god wouldn't understand. Somebody explain to me what that shit actually meant (Somebody said: My sigma self had to miss my gooning streak to watch this not so skibidi post. WHAT DOES THIS MEAN?)