What does the GSM AT+CSQ command output signify?
According to this your Received signal strength is -63dBm.
RSSI (=Received signal strength indication). As you say, you can receive RSSI without SIM (SIM IDs you in the network as a user so you can make calls. All the radio capabilities are in the phone, you can check the signal strength without the SIM).

For checking the available networks use:

AT+COPS=?
+COPS: [list of supported (,long alphanumeric,short alphanumeric,numeric[,])s][,,(list of supporteds),(list of supporteds)] +CME ERROR: