The term “proximity effect” most often refers to a change in the frequency response of a directional microphone as the sound source is brought into closer proximity to the mic. The net result of this is a rise in the overall bass response. For decades, radio and TV announcers exploited this effect, moving in closer to the mic to make their voices seem richer and deeper. Omnidirectional mics, because of the way they are designed, do not respond in the same manner, so if you wanted to close mic an instrument without boosting the bass, you’d probably choose an omni mic.
But the proximity effect can be put to good use in recordings. For example, moving a cardioid condenser mic closer to a bass guitar’s speaker can cause low frequencies to be boosted. In the studio, this means that a huge speaker stack may not be necessary, as a single 12-inch speaker can be made to sound huge, simply by moving the mic in close. In the same manner, singers with slightly weaker voices can be made to sound richer and fuller by simply singing close to a cardioid condenser mic. A good pop filter will probably be required in such instances to avoid nasty plosives.