This module allows you to use in-memory data structures which are larger than available memory and so are kept in swap. If you try this in normal OCaml code, you'll find that the machine quickly descends into thrashing as the garbage collector repeatedly iterates over swapped memory structures. This module lets you break that limitation. Of course the module doesn't work by magic :-) If your program tries to access these large structures, they still need to be swapped back in, but it is suitable for large, sparsely accessed structures.
Secondly, this module allows you to share those structures between processes. In this mode, the structures are backed by a disk file, and any process that has read/write access that disk file can map that file in and see the structures.