[unreal 5.1][physic] how to make a box with a mass kg

Hello I begin with Unreal 5.1

I try to understand the physic engine and I would like to make a simple box with a gravity mass in Kg that fall on Y axis.

And so modeling → box - Details-> Physics physics simulate Mass I give some kg but what I must to do after that ?

Sometime I see tutorial where the instructor give a color with a blueprint and I think Why ?