This meter has no units so what is it measuring? I'm assuming that it is a voltmeter measuring the emf generated by placing your hands on the different metals.
https://www.exploratorium.edu/snacks/hand-battery This site explains that the electrolyte is actually the sweat on your skin which allows the metals to give and receive electrons. The aluminium is more reactive so it finds it easier to lose electrons to form positive ions. Copper finds it harder to lose electrons so there is a net flow of electrons from aluminium to copper.
https://www.mpoweruk.com/chemistries.htm
