In IEEE 802.11 WLANs an optional mechanism allows the stations to reserve access to the channel by small control packets. The sender transmits first a short RTS (request to send) control packet, indicating the total time required to transmit the data. When the access point receives the RTS packet, it responds by sending a CTS (clear to send) packet including again the required time for the complete transmission.
This scenario shows the benefits of the RTS/CTS mechanism in a special scenario, the so-called hidden station problem, by an analogy where
a fire department represents the access-point
two burning houses symbolize hosts in the WLAN
The two houses are out of range to each other, thus they can only communicate with the fire department, and don't hear the transmissions between the other house and the fire department.
In the analogy, both houses catch fire and they both send simultaneously a "HELP!" message to the fire department. Due to the signal collision at the fire department it's impossible to understand a single emergency call and both houses burn down.
The second scenario is quite similar, but this time the first burning house sends a RTS message first. The fire department receives the RTS and answers with a CTS message. This is received by all houses in the range of the fire department and they are not allowed to transmit anything for the reserved duration. So the emergency call from the left house is received by the fire department, and the one from right house in a second step, too. Both houses can be saved!