The goal is to clear the blocks from the board by pushing into them. Sometimes blocks are piled high and must be hit more than once. This is what they look like:

One block
Two blocks
Three blocks
Start by choosing an empty area...
...and click to place your circle
Click to move towards blocks. When you hit them, you push them and remove one level (so one block becomes empty). You can only move horizontally or vertically. There must be a gap between the circle and the block, like this.
If you are right up against a block, you can't push it.

Keep moving until the board is clear!