Comma-separated list of IP addresses or host names of your caching proxy servers. The port those servers will be contacted defaults to 6081, you can specify a different port with :<port>.
However when using varnishlog (varnish 4.x) i can not see incoming PURGE requests if the Port is omitted in the config.yml, even when using standard port 6081. When i supply the standard port in the config, the PURGE requests show up fine and work.