Null launcher is a launcher that does nothing. Yes, really. It's useful when combined with Wave Launcher, Swapps, and similar.
After installing, press your home button. Then, depending on your android version, either check the "Use by default" box and tap Null launcher, or tap Null launcher then tap "Always."
If you've accidentally switched to Null Launcher with no way to get out, pinch-zoom on the wallpaper and you can change back to your old launcher.