Monitor standby is a feature in the GX series and SL-30 that does not exist in the GNSx30 series.
It allows simultaneous reception of the Active and Standby Freqs for either nav or com so that you can, for example, pick up Atis while staying on Center.
You can only transmit on the Active. So, it's like a radio and a half without needing an Audio Panel.
For NAV in some installations you can use the standby on the ondeck CDI for instersection crossing (This is true of the Garmin SL-30 VAL NAv2000 and I generalize to say it's available on other radios)