The blocking feature in Ansys ICEM CFD provides a projection-based mesh-generation environment. All block faces between different materials are projected to the closest CAD surfaces. Block faces within the same material can also be associated to specific CAD surfaces to enable the definition of internal walls. Generally you do not need to perform any individual face associations to underlying CAD geometry, which reduces the time for the mesh generation.
The blocking step is used when a structured, hexa-mesh is desired in one or more parts (quad mesh if 2D parts). A premesh is generated in the blocked regions which can be refined and improved on a block-by-block basis. The premesh data is converted to structured or unstructured mesh data before it is merged with mesh data from other parts or passed to a solver.
There can be multiple Blocking strategies, depending on the topology you are working with:
Typically you take a top-down approach, in which you first capture the outer geometry and then split, delete, and merge blocks to capture the minor geometry. The volume can be filled by a variety of methods including mapped hexa blocks, swept blocks, or unstructured mesh zones (which can be filled with a variety of unstructured methods available).
You may be able to use a bottom-up approach, in which you start by creating a 2D blocking, add blocks to capture more detail, and then extrude the 2D blocking to create the 3D blocking. Again, the volume can be filled by a variety of methods.
For further information on blocking, see Blocking Strategy in the Hexa Meshing chapter.