RSSI stands for 'Received Signal Strength Indicator'. It's a measurement of how well your device can hear a signal from an access point or router.
When installing your NoiseAware sensors, your indoor should have a max RSSI of -60 to -70 and your outdoor should have a max of -100. The closer you get to -35, the better! -35 indicates that your sensor is basically on top of the router.