Please read OpenFOAM - Basics before reading this post
blockMesh is a mesh generation utility supplied with OpenFOAM. In simple word it generates mesh for the CFD analysis. In the last post it was said that the basic OpenFOAM case directory consists of three folders namely 0, constant and system. The blockMeshDict file in the constant/polyMesh directory of the case file contains the parameter of the geometry and the parameters required for meshing the surface. Now go to the forwardStep directory which is discussed in the previous post and open the blockMeshDict file. In the file u scan see may sections like FoamFile, vertices, blocks, edges, etc.,