In order to maintain compatibility between newer and older technologies for example 802.11b (HR-DSSS) and 802.11g (ERP-OFDM) a mechanism was devised to allow the 802.11b device to understand the 802.11g device needed to transmit that doesn't significantly lower the overall data rate of the 802.11g client (ie force the 802.11g client to use legacy data rates.)
The 802.11g device will send an RTS/CTS frame sequence (Request To Send/Clear To Send) that should be heard by all stations, it may also use only "CTS-to-self." This sequence will be understood by the 802.11b station that will read the duration field from the frame and set its NAV timer to hold off the medium until this timer expires. This "protects" the medium and allows the 802.11g to transmit unhindered.
An AP will notify all clients within its service area that there are 802.11b devices present via a bit set in its beacons