MDB Tools is composed of three libraries, a set of command line utilities and a graphical interface.
libmdb is the lowest level library. It handles the details of reading from and writing to the MDB file format. Atop this library sits all the other functionality of MDB Tools.
A complete description of libmdb will be available in the forthcoming MDB Tools Programmer's Guide. For now you can look at the utility programs for a general idea of how to use the API, it's fairly straightforward.