LQI and RSSI together tell you how well your sensor is connecting to the system. Both of these are relative values but in general use the following:
LQI measures signal quality. It ranges from 255 to 0. 255 is best, 200 is ideal. Much below that you may see issues.
RSSI measures signal strength. It -100 to 0, 0 is best. 0 is best, Below -75 can be problematic.