kernel question - promiscous mode what is that?

Hi Susers I just checked my log and noticed the following "...eth0: Promicous mode enabled ... eth0: device eth0 entered promiscous mode ..eth0: device eth0 left promiscous mode" inbetween entered and left promiscous mode I can see that a client has accessed his pop account. Anyone have any clues what this mean? Have I been hacked? Thomas