01 BASIC Bridging
At its simplest, the Bridge tool creates geometry between two selected and unconnected elements. These can be vertices, edges or polygons, ideally with the same edge count. Using Bridge with polygons to create a new length of geometry can remove the end polygons, creating a tunnel. Using the Bridge command with edges can be an excellent way to retopologise a scanned model into a mesh pattern, which works better for animation.