Play the drums as the track plays. When a note reaches the strike line, hit its lane. On a computer that's A S D J K L and SPACE; on a phone, just tap the beats as they land. There is also an osu-style Cursor mode for mouse players.
Each lane is one drum. When a gem reaches the glowing strike line, hit that lane. On a computer, press its key; on a phone, tap its pad at the bottom. Tighter timing scores more and builds your multiplier; miss and your rock meter drops.
Higher difficulties add more of the kit and more notes, from a simple two-piece backbeat up to the full pattern. Clear a difficulty with 80% accuracy to unlock the next one.