A block is a an elementary geometric solid that has specific geometric constraints oriented for meshing.

Tetrahedral Block - is a block with constraints for tetrahedral meshing. It has 4 faces and 3 edges on each face.
Hexahedral Block - is a block with constraints for hexahedral meshing. It has 6 faces and each 4 edges on each face.
Block Compound - is a compound composed of blocks only.

The functionalities that allow building blocks are available from the main menu via New Entity / Blocks item.