A slipmat is a circular piece of slippery cloth or synthetic materials disk jockeys place on the turntable

Phonograph

The phonograph record player, or gramophone is a device introduced in 1877 that has had continued common use for reproducing sound recordings, although when first developed, the phonograph was used to both record and reproduce sounds...

 platter instead of the traditional rubber mat.

Unlike the rubber mat which is made to keep hold the record firmly in sync with the rotating platter, slipmats are designed to slip on the platter, allowing the DJ to manipulate a record

Gramophone record

A gramophone record, commonly known as a phonograph record , vinyl record , or colloquially, a record, is an analog sound storage medium consisting of a flat disc with an inscribed, modulated spiral groove...

 on a turntable

Phonograph

The phonograph record player, or gramophone is a device introduced in 1877 that has had continued common use for reproducing sound recordings, although when first developed, the phonograph was used to both record and reproduce sounds...

 while the platter continues to rotate underneath. This is useful for holding a record still for slip-cueing

Slip-cueing

Slip-cueing is a turntable-based DJ technique that consists of holding a record still while the platter rotates underneath the slipmat and releasing it at the right moment. This way the record accelerates to the right speed almost immediately, without waiting for the heavy platter to start up...

, making minute adjustments during beatmatching

Beatmatching

Beatmatching is a disc jockey technique of pitch shifting or timestretching a track to match its tempo to that of the currently playing track e.g. the kicks and snares in two house records hit at the same time when both records are played simultaneously...

 and mixing and pulling the record back and forth for scratching

Scratching

Scratching is a DJ or turntablist technique used to produce distinctive sounds by moving a vinyl record back and forth on a turntable while optionally manipulating the crossfader on a DJ mixer. While scratching is most commonly associated with hip hop music, since the late 1980s, it has been used...

. They are also very commonly used simply as decoration for when a record isn't on the turntable.