Perpetual swap contract also commonly called "perp", "swap", "perpetual" or "perpetual future" in crypto exchanges nomenclature is very similar to futures contract, but does not have expiry date (hence perpetual). In order to ensure that the perpetual swap contract price stays near the spot price exchanges employ mechanism called funding rate. When the funding rate is positive, Longs pay Shorts. When the funding rate is negative, Shorts pay Longs. This mechanism can be quite nuanced and vary between exchanges, so it's best to study each contract specification to learn all the details (funding periods, mark price mechanisms etc.).