It usually means you can sense the acoustic space round the instruments and singer, which enhances the 3D effect. This can be real ambience, e.g.: on an acoustic instrument such as a piano or guitar recorded with more than one mike in an environment with natural reverberation, or artificial, added by the producer during the mix to a synthesized sound or to a natural sound (eg vocals) which was recorded in mono in an acoustically-dead space (such as a vocal-booth).
When recording drums, as well as close-miking the kit, the producer/engineer will often place one or two mikes away from the kit to get a sense of the space around it. These are often called ambient mikes, as they're there to give you an impression of the size of the kit and ambience of the drum-room.
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