What is this 2x4 brick with a 2x3 rounded plate with hole attached? I found this LEGO brick and have no idea what it does, it’s name, or which set it is from. It is a 2x4 brick with a 2x3 base plate with a rounded end and a hole in it. 

Can anyone tell me about this brick?
It's a magnetic base that minifigs are attached to.