Drivers (more precisely: content part drivers) are pieces of code used in conjunction with content parts. They are responsible for building the editor and display as well as handling importing and exporting of a content part.
Note that the same part can have multiple drivers.
This page and many others here are part of the open source, portable package of Orchard goodies: the Orchard Dojo Library. The Library is freely accessible by anyone; just go to its repository to download its full content (you can even download the textual content of the Library as a big concatenated document in HTML), file issues or fork it! You can even edit it online on Bitbucket, just click on the Edit link in the upper right corner!