Snaps objects to a grid when placing and displays a build grid around it (unless you hold ctrl).
You can configure whether a build grid displays at all, as well as the size of the grid (it defaults to 10 in each direction). Keep in mind that going all the way up to 50 will almost certainly cause some lag when you're placing things.
Credits to zkm2erjfdb and Levorto for writing the original single-player versions (Architectural Geometry and Assisted Geometry). This mod is a replacement for those mods; if you have one of them enabled as well, unpredictable things will happen.