Within X Rebirth, player-created content that changes the way the game works is called an "extension" and those that are in-use can be seen listed in-game via the main menu. Extensions are equivalent to within earlier Egosoft games, where player-created content took the form or "scripts" or "mods". Given that past familiarity, X Rebirth extensions are sometimes called mods and their creators are still refereed to as modders.
Please note that the use of extensions or save game edits can break games either of themselves, after future game updates, or via conflict with other extensions. Consequently, Egosoft are not generally able to support such modified games (see here for further details). However, do feel free to post any problems you may experience in appropriate threads on the X Rebirth - Scripts and Modding forum as the creator or others may be able to help you.
The "Finding mods" item provides tips for players, which basically includes everyone. The "A collection of modding resources" item lists Egosoft forum threads, and begins to pull together various tips dotted around the forum. The latter items have been created by Egosoft with the aim of providing help and information to modders.
- Egosoft's Technical Engine Development faq / blog (link).