My friend owns a 1996 Subaru Legacy Outback. I recently had to drive the car, but when I tried to start it it wouldn't start. When I removed the key a loud clicking sound came from somewhere behind the wheel. When I called my friend they told me that this happened all the time, I just had to unscrew and rescrew a screw on the battery (the green one in the top right of the attached picture). I did this and the car started.
I tried googleing this issue, but wasn't sure what to search for. Can anyone tell me what the problem might be, and if this is safe to drive?