RTS/CTS (Request to Send / Clear to Send) is the optional mechanism used by the 802.11 (Standard WiFi) wireless networking protocol to reduce frame collisions introduced by the problem known as hidden node problem.
The WiFi Advanced configuration parameter RTS Threshold specifies how big packets must be for the SM to utilize the RTS (Request to Send) packet. When Uplink RTS/CTS Enabled the SM/Client must send an RTS frame and, only upon receiving a CTS frame from the AP can it transmit uplink data over the wireless link. This allows the Access Point to schedule uplink transmission for each client helping avoid packet collisions (and resulting retransmissions / low throughput).
RTS and CTS packets take channel time away from data transmissions. If a data packet is small, then it is more efficient to just send the data and potentially collide with another device’s transmission and retransmit the data packet again than send the RTS and CTS.
If a data packet is large, then it is more efficient to send RTS and CTS packets first to prevent a collision with another device's transmission as large data packets that collide will cause the channel to be unavailable for a large amount of time.
A good starting point for configuring the RTS Threshold parameter is 500 bytes. Adjust this value incrementally and observe network performance/collisions to avoid drastic changes in the operation of the wi-fi network.