Stereo Width is a term that explains where various pieces of a track sit in the audio listening field. A "wide," vocal for example will include elements hard panned to the Left and Right, while a vocal that isn't wide will purely sound like it is located in the center of the listening field.
Stereo Width can add an incredilble amount of depth and interest to your track, but it can also cause significant phase problems if done incorrectly.
With eMastered's Stereo Width feature you can experiment with hearing your song wider and more narrow, and you won't need to worry about ruining the phase at all.