While holding down the ⌥ key when clicking on the wifi icon, you can learn more about the strength of the current wifi connection.
RSSI or Received Signal Strength Indication can go from 0 to -100. Generally the higher (closer to 0) the better, and the closer to -100 the worse. In telecommunications, RSSI is a measurement of the power present in a received radio signal (cf. Wikipedia).
The number to pay much more attention to is the Transmit Rate. This value is the rate of which information is processed by a transmission facility. The transmit rate will drop if you have too much noise or interference or if you are too far away from the base station.
0 - No connection 6 - Half 802.11b 11 - 802.11b 54 - 802.11g 130 - 802.11n on 2.4 Ghz 300 - 802.11n on 5.0 Ghz