It's quite simple actually. In nature, you always hear a sound no matter what direction it came from with both ears. The sound usually reaches one ear with a little time difference and lower level. We humans use this information to locate where the sound came from.
With headphones, the left channel can be heard in the left ear only, and vice-versa of course. This unnatural stereo separation usually causes fatigue (unless you're used to it) and also creates an unnatural/artificial soundstage*. By adding some crosstalk with time delay to the stereo signal (= crossfeed) you reduce this unnatural experience and get closer to what your hearing/brain expects.
*) feels like sounds are coming from the inside of your head
If you ever sat in front of a good stereo speaker setup and closed your eyes you should know what a real soundstage sounds like.
Depending on the DSP you're using you can get close to that experience with headphones, but it requires some time and tweaking.