Hello,
as Decals are still a daily build feature, we did not yet create any tutorials or articles on it. I believe the best practice would be to try and play around with it to understand.
From my limited understanding - A material/map applied to Decal is projected onto a surface of an object it overlaps, ideal for e.g. graffiti on a wall or even something displaced, such as man-hole covers or refractive, such as windows.
If you need further info, feel free to ask here.
Thanks,
Jan