"Mon" key would be the one you push to "monitor". It either opens up
the squelch if you are on simplex, or checks the input frequency if you
are on a repeater split. On the C510 is right below the PTT button.

Sorry to report that my radio must be completely different from yours; it
has no such animal on it. There is a rotary control to break the squelch,
and a function key on the keypad lets you check an input frequency...