Backing, Support, Backer, Supporter
noun
The rear shield is the shield that protects your back. This is your backing or support. So, not a literal shield, but the kind of protection that you want to have anyways.
うしろだて
The reading comes from 後ろ and 盾 put together. Watch out for the rendaku.
私は友人の後ろ盾に勇気づけられました。
I was encouraged by my friend’s support.