Electrical – this electrical panel for

It's how the city reads your water meter. You should be able to trace the wires back to your water meter, or if that isn't feasible, look at your water meter and you'll probably see a device on it with two wires of the same color.

I doubt that "button" can actually be pressed, rather it's the contact point where the meter reader places their probe to take the reading.

_{(source: thetimes-tribune.com)}

A main panel (or service entrance panel) is simply a panelboard that contains the main service disconnect for a property -- this can be up to six circuit breakers in a rule-of-six (split bus) panel, but is more commonly a single main circuit breaker or fused disconnect (such as a pullout).

Or in other words: the service from the electrical utility enters the main electrical panel for a property, and all other panels on that property are subpanels, as long as there is not a second utility service or a transformer on the property. There are also other special cases such as multiple section main panels, but they're beyond the scope of this answer.