Your bottom game is your ability to attack from positions where your opponent is on top: guard, mount escape, bottom side control. A strong bottom game makes you dangerous even when behind.
Control their sleeves, collar, or pants.
Use your feet and hips to create separation from their weight.
Break their posture and balance constantly.
From good guard, you're always one grip away from a sweep or submission.
Sweeps transition you to top position when your guard is being passed. Master the fundamental sweeps: scissor sweep, flower sweep, hip bump, and leg drag sweep.
Bottom positions require constant activity. Keep moving your hips, constantly threaten with sweeps/submissions, and never let them settle into heavy pressure.
A complete bottom game makes you unpassable and dangerous. Build it and you'll win matches from any position.