The Chunk0.def file is a file used by WWE 2k19 and WWE 2k18 to point the game to the .pac files it should load on startup. Any .pac files that isn't listed in the Chunk0.def won't be read by the game. Earlier games don't utilize this file.
Relevance to Modding
When adding mods in WWE 2k19 or WWE 2k18, you always have to add an entry with the relative path to their .pac files to the Chunk0.def file. One can do so manually with any text editor (Notepad, for example), but the easiest and most reliable way to do so is using Custom Character Tools 2019's Chunk Tools-> Regenerate Chunk0.def or Custom Character Tools 2018's Utilities -> Regenerate Chunk0.def menu option after you have installed the .pac files (note that if you use Custom Character Tools to install chXXXYZ.pac files, the Chunk0.def file will be updated automatically and thus you usually won't have to regenerate it manually).