You remove blocks by filling an entire horizontal line, a vertical line, or one of the nine 3x3 squares.
High-level players often emphasize a "Zen" balance between immediate clearing and long-term planning:
Keep at least one 3x3 area clear at all times, as most game pieces can fit within this space, preventing an early game over.
Do not leave gaps while waiting for a "perfect" piece (like a long straight bar); it is often safer to clear what you can immediately.