"It's about to go down" is an expression that means something is about to happen either good or bad. For example:
If a angry man enters a room holding a baseball bat, I will tell my friend Let's get out of here, it's about to go down" meaning there is about to be trouble.
If a hungry man walks into a buffet restaurant, he might say to his friends "It's about to go down" meaning I'm about to eat a lot.