MIN_JITTER and MAX_JITTER specify minimum and maximum size of RTP Jitter Buffer, which is used on softphone side.
Using of jitter buffer allows to increase quality of incomming audio on bad Internet connection (when packets are received in different order, than they were sent originally).
Example:
!define MIN_JITTER 0
!define MAX_JITTER 1000
Setting too big jitter buffer is not recommeded because this will increase delay in audio.
The softphone adjust automatically jitter buffer size depending on the packet arrive time statistics.
When Internet is good, then actuall jitter buffer will have size as MIN_JITTER value (in milliseconds).
When Internet is bad, then actuall jitter buffer will have maximum size MAX_JITTER.