As said before, libraries are ordinary pieces whose waveforms and envelopes can be referenced from within another piece. They can be used as "sound banks" that you can reuse across a variety of pieces. A default library is always added whenever you create a new piece, but you can add as many as you want. Libraries can also reference other libraries.
Use File > Insert Library to insert a library in your piece. The library's envelopes and waveforms become visible under their own sections in the waveform and envelope editors. You may want to click on the library's name to reveal its content if it's hidden, or hide it by clicking again on the library's name.
When you move a piece file, you should always keep the library files it references together with the piece, so that those files can be retrieved when you open the piece again.
A library is a regular piece. To make changes, simply open it as a piece (using File > Open Piece), add or remove envelopes and waveforms, or edit them at will. Then, save the library, and reopen all the pieces that use this library to see the changes reflected.
click on the library name, either in the envelopes list or the waveforms list, and press the "-" button.