Highlighting an area/rectangle on the floor

In my game I want the player to be able to highlight an area on the floor.
When he clicks and holds the mouse while moving it, I want to hightlight that area he is “painting”.
For example to show where something would be built.

Something like this:


Unfortunately I have no clue how to achieve that, as I have basically no experience with materials.
Or do I even need a material?

Can you guys guide me into the right direction?