Music playback time is the officially quoted longest time that a single battery charge will last when you use the phone as a music player only.
Usually those numbers are only achievable when the phone is set to Flight mode (i.e. all transceivers are off) and the headphones are used (as opposed to the loudspeaker). These numbers are best used as reference when comparing phones from the same manufacturer because battery life measurement methodology will probably vary from manufacturer to manufacturer.