Modding refers to modifying Dwarf Fortress' files (or, indeed, the files of any other game) to change some element of game play, usually by a third-party developer or the end user. In Dwarf Fortress, this usually refers to changes to the so-called raws, the game files that describe a wide range of elements, ranging from what plants can be milled, to what kinds of rock exist, to how empires are formed.
Most editing involves adding or removing "tokens" from the raw files. The tokens are the actual parameters for each item, and that article is a good place to start.
This category has the following 9 subcategories, out of 9 total.