The 802.11 standards body introduced Quality of Service via Task-Group 802.11e. The 802.11e standard introduced the concept of prioritized Traffic and Transmit Opportunities (TXOPs).
The Transmit Opportunity for a Station meant that the 802.11 transmitting station would be allocated a particular time period (relative to the QoS access category) for the transmission rather than having to contend for each packet transmission as shown above.
This allowed the 802.11 station to transmit multiple packets to the Access Point in a Transmit opportunity. Other 802.11 Stations in the network would back-off for the duration of the TXOP by reading the duration information in the RTS/CTS frame transmission by the transmitting Station and Access Point
The TXOP mechanism is shown below
The above mechanism allowed the removal of back-off for each Data packet as was the norm in the Legacy 802.11 implementation and hence provided MAC efficiency.