The Fill Hole action creates a new face that fills any holes that touch the selected edges.
From the main menu, select Tools > ProBuilder > Geometry > Fill Hole.
Enable the Fill Entire Hole option to fill the entire Mesh opening. This is the default.
If you disable this option, ProBuilder tries to build a Mesh between the selected open edges. For example, if you have a missing quad, you can select two adjacent edges in order to create a triangular polygon that covers half of the hole.