Saltwater battery explaination

Electrolysis.

Cheers, Pete.

You need to study chemistry to get the details, but the short answer is that the aluminum is being oxidized, and the energy from this reaction is driving electrons around the external circuit to the copper. This works with any pair of dissimilar metals in a conductive solution, with the more active one being the cathode (negative) terminal. The greater the difference, the greater the voltage. A little work with a search engine will turn up as much detail as you can use.

The Cathode, Copper is the positive terminal. The Anode, Aluminum is the negative terminal. Remember, electrons collect on the Anode making it negative. Aluminum Ions leave the Anode and go into solution. Electrons travel in the external circuit from the Anode and come to the Cathode where they are "emitted" into the solution to combine with the Ions there.