Narrow mixes keep most sounds near the centre; wide mixes spread instruments and effects to the sides. Bass and kick are usually kept central for punch and mono compatibility, while pads, reverbs and doubled guitars are spread wider.
Phones often play back in mono or near-mono, so very wide effects can vanish on small speakers.
Using it in a prompt
Ask for "wide, immersive mix" for ambient and cinematic music, or "focused, centred mix" for content played mostly on phones.