Because a perpetual never expires, something has to stop its price from drifting away from the underlying asset. That mechanism is the funding rate.
How it works
At regular intervals one side of the market pays the other. When the perpetual trades above spot, longs pay shorts; when it trades below spot, shorts pay longs. Those payments nudge the contract price back toward spot.
What it means for you
If you hold a position through a funding interval, you either pay or receive funding. In calm markets it is near zero; in strongly trending markets it can add up, so factor it into any longer hold.