Acá tienes una explicación:
"backlog is the number of connections allowed on the incoming queue. What does that mean? Well, incoming connections are going to wait in this queue until you accept() them (see below) and this is the limit on how many can queue up."
Extraido de:
http://www.beej.us/guide/bgnet/output/html/singlepage/bgnet.html#listen
Slds.