What do the question marks represent?

What do these question marks represent? How can I see what they are hiding? Obviously I have solved and unlocked the level, but the question marks still remain and leave me with a few question marks above my head.

Solution 1:

The Question Mark means that the Block That Matters was not found in this particular level. A Block That Matters is a block that makes reference to an existing game made of blocks/tiles.

Each level in the main adventure mode contains a chest. If you manage to drill this chest and bring it to the exit, you will discover a Block That Matters!

Then, this block will appear in the level selection menu, and the "???" will be replaced by some info about this Block That Matters (name of the game, makers, platforms, etc).

Solution 2:

To add to @Guillaumes answer - actually if you are (unlike me) patient enough to get to the fourth level (third + the tutorial level) without having to know every single enigma of the game asap, you will encounter your first non-hidden chest with an explanation about what it is, and you will figure all this out on your own. But if you're reading this, I guess you're not the patient one, are you? :D