You can press Delete or use the ERASE command to remove mesh faces. The removal leaves a gap in the mesh.
If removal of a mesh face creates a gap, the mesh object is not “watertight.” It can be converted to a surface object, but not to a 3D solid object.
If a mesh object is not watertight due to gaps, or holes, in the mesh, you can make it watertight by closing the holes. The cap, or new face, spans the boundary formed by the mesh edges that you specify (MESHCAP).
This process works best when all edges are on the same plane. The edges you select as boundaries cannot be shared by two faces. For example, you cannot close the center hole in a mesh torus.