IEEE 802.11 WLANs use CSMA/CA for media access. Additionally, an optional mechanism allows the stations to reserve access to the channel by small control packets.
In this case, the sender transmits first a short RTS (request to send) control packet, indicating the total time required to transmit the data and the acknowledgment packet. 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.
The RTS/CTS mechanism informs all stations in the range of the sender and the access point (receiver) about the planned transmission and instructs them not to send for the reserved duration. Thus it serves two purposes:
Since the RTS and CTS packets are short, a collision will only last for the duration of the short packet. The following data and ACK packets are transmitted without collision
The hidden station problem can be avoided, since all stations in the range of the receiver are informed about the transmission and wait until it is finished
You can see how the RTS/CTS mechanism works, if you press "play" and then interactively select the stations that should transmit a packet. Packets are always transmitted from the stations to the access point and acknowledged by this one.