A block library is simply a collection of blocks usually subdivided into categories for ease of finding and using.  Example:  A block library for someone in the field of chemical engineering might have a master folder called Symbols.  Under this folder would be sub-folders for such things as Instrumentation, Valves, Computer Control, Equipment, Piping and Labels.  Each of these folders will contain the actual blocks which are just .dwg files.  Example: The Valves folder might contain block drawings of a ball valve, spring loaded valve, plug valve, check valve, knife gate, gate valve, etc.  Do you understand?

Either create a custom tool palette and drag/drop their blocks onto it or use the Design Center to drag the blocks from their drawing into your drawing.