WiFi.RSSI() returns the signal strength of a Wi-Fi network from -127 (weak) to -1dB (strong) as an int. Positive return values indicate an error with 1 indicating a Wi-Fi chip error and 2 indicating a time-out error.

int rssi = WiFi.RSSI();
WiFiSignal rssi = WiFi.RSSI();

Since 0.8.0

WiFi.RSSI() returns an instance of WiFiSignal class.

WiFiSignal sig = WiFi.RSSI();

If you are passing the RSSI value as a variable argument, such as with Serial.printlnf, Log.info, snprintf, etc. make sure you add a cast:

Log.info("RSSI=%d", (int8_t) WiFi.RSSI()).

This is necessary for the compiler to correctly convert the WiFiSignal class into a number.