From the Super Mario Wiki
The Flip-Flop Block is a common block found in Mario & Wario. These blocks appear in every normal level. Wanda has to use them to guide the character to the goal as Wanda can make it appear or disappear. Invisible panels still exist in the levels as they appear as dotted boxes. There is also a smaller version of a Flip-Flop Block, as well as a version that is totally invisible unless flipped solid (which have an exclamation point on their design).
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