Battery wires?

Today in the morning my car started perfect and I drove about 3 miles and at a local shop I turned the car off and went to shop, after I returned and sat in the car and tried to self-start the car would not start, what I faced was when I inserted the key all the lights glowed but when I wanted you start the car the lights vanished and all the electrical systems went off including car locking system I opened the bonnet and realized that a wire on positive battery terminal was loose and I pushed it to make it contact firmly after that my car started. The question is, I don't know what that wire is I need help in this regard. The pictures are attached also. That third wire goes from the battery positive terminal through the fender and then inside the passenger's side door.