promiscuous mode enabled

Jan 28 12:04:53 bsd-test kernel: em0: promiscuous mode enabled
Jan 28 12:04:53 bsd-test kernel: em0: promiscuous mode disabled

# ifconfig -a
em0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu 1500

to answer the rest of your question:

promiscious mode means the interface listens to all traffic that comes across it; various applications (particularly sniffers,) place an interface in promiscous mode in order to sniff network info (notably tcpdump and any other apps that use tcpdump's functions, like ethereal).