Progrids is a unity package that let's you set up a 3d grid snapping, that is you can set the value that you want your assets to snap to, it's extremely helpful when setting up your level. you can read about it here:
About ProGrids | ProGrids | 3.0.3-preview.6 (unity3d.com)
However currently adding it to your project is not very straightforward, it's in prelease and does not come up in unity registry under the package name, the easiest way is to add it by name.
Go to top menu select Window-> Package Manager
with your package manager open in the top left corner click, the little plus sign and select add package by name or from git URL.
Next type in com.unity.progrids and hit the add button
and that's it you'll have successfully added progrids to you project.
