I can't find any information about it except one sentence in documentation - Set number of packets to buffer for handling of reordered packets.
Can it help with unstable network or stream? What default value is and what value should be set and when?
Packets may be received out of order, so before they can be passed onwards, they need to be reordered. Default jitter buffer size is 500. This option allows you to specify it manually. Higher values will lead to greater latency. Only applicable for reception over UDP.