Libraries contain various items such as envelopes, waveforms and patterns that you may want to use in several pieces. A piece can reference multiple libraries which are pieces like any other. When the piece is loaded, the definition of the envelopes and other items will be taken from the library's file instead of searched in the piece.
You can insert predefined envelopes from another piece by using File > Insert Library... and opening another piece that contain the desired envelopes. It is a good idea in general to store waveforms and envelopes in separate location from pieces, to allow easy reuse. Envelopes in libraries are referenced by their names, which means that you can substitute a library for another or edit the library and reload the piece to benefit from the changes made to the envelopes.