The meter display has the following arrangement
left bar: Momentary Loudness value
right bar: Short-term Loudness
text overlay: Integrated Loudness and Loudness Range (LU) values, Gated indicator lights red when gate is active
Clicking the button pauses measurement; clicking again resumes it. This allows you to make adjustments without affecting Integrated Loudness, instead of having to start all over again.
Clicking the button resets the meter to its initial state.
Don’t forget to reset the Loudness meter if you’re starting playback of a new track, as Integrated Loudness, by design, measures the overall Loudness since the last reset. Otherwise you’d be measuring the overall Loudness of the combined tracks, which is probably not what you want.