It is a special library created to manage pins of microcontroller in any embedded system.
This is the whole ideology where the connection between a created software has to be independent from a physical pins mapping. You can, for example, connect data pins to your LCD in any order to your port. It can be just a straight mapping (D0 to pin 0 of port, D1 to 1, ..., D7 to 7) or it can be a total mess. The connection can be even distributed between different ports if the design requires it.
I invite you to read the documentation on http://k-el.com/dochost/.
Project is documented using Doxygen and whole documentation is inside the sources.