From the Super Mario Wiki
The Checkpoint Block is a type of block that Wario can hit in Wario Land: Shake It! to save his progress up to that point for every level. They almost always appear right before the imprisoned Merfle. They are very useful to whack, for should Wario either lose all of his HP or not make it back to the start of the level in the given time, he will pop out of that block if the player chooses to retry the level. Also, if Wario does not collect enough coins or treasure to his liking, or should he take damage on the way back with a Merfle, or any other type of mission that needs to be completed, he can restart the level from the start of that block at his will. However, some levels have a mission that will state that the player should not hit the Checkpoint Block, and they will have to complete the level without hitting it to clear the mission.