Understanding sound stage on headphones
If you listen to a track on two pairs of headphones to compare its sound stage, and the first pair sounds like everything is happening in your head, versus the second that sounds less in your head, the second has a better sound stage. If you listen to the same track and find that sounds positioned far left or right are more evident on one pair of headphones than the other, than these headphones have a better sound stage.
This is very important if you are using your headphones to mix music. When positioning instruments, sounds and sound effects in the stereo image, the headphones with more sound stage will help you make a better mix! Open back headphones are usually better in sound stage than semi or closed back.