67 barracuda Wiring mess!

You have a shop manual? That should be the no1 project on your list. Unfortunately, some of the Barracuda wiring is a little special, the Rally cluster, for example.

I'd get an electronic pdf manual, they are for sale here and there on the www.

"Somewhat useful" diagrams are here:

http://www.mymopar.com/index.php?pid=31

The thing you are asking about is your starter relay. The two "push on" terminals are the coil terminals of the relay. The yellowish wire comes from the "start" contacts in the ignition switch, and the remaining push on connector runs down to the neutral safety switch on your transmission (automatic).

The big stud is a battery junction as well as one of the relay contacts

The "square" looking terminal is the remaining relay contact (switch) and fires the starter solenoid when the relay is engaged.