There is a lot to know about the Xposed Framework, and a lot that could go wrong — use this guide to get started
So you have an Android phone because you know you can bend and twist just about everything in the software until you make it just right. You've done the custom ROM thing, but there are still things you want to tweak. That's where the Xposed Framework comes into the picture.
Xposed is a base system that allows you to download modules, each of which can make one or more changes to the UI of your device. Things like adding the three dot menu to every app, or enabling the full 20MP sensor for Superior Auto in Sony cameras, or any manner of changes. There's a whole list of modules you can download and install at the Xposed repo site.
Because it's such a powerful tool, there are also a million things that can go wrong if you just jump in without knowing what you're doing. You should definitely read all the documentation, but AC Forums Ambassador Golfdriver97 has also whipped up a really handy getting started guide to keep you pointed in the right direction. Read it. Twice.
Half the fun of hacking your phone is fixing it when you've broken it, but some tools are a bit more complicated than others. Xposed is powerful, and guides like this one are a great way to keep things going smoothly.