Basis
The basis is the difference between a futures price and the spot price of the same asset (basis = futures price minus spot price). It is usually small, because traders profit from any large gap and close it.
A positive basis means the future trades above spot; a negative basis means it trades below. For a dated future the basis shrinks to zero at expiry, because the contract then settles at the spot price. For a perpetual future the basis is kept small by regular funding payments between longs and shorts.
The size of the basis tells you how eager the leveraged side is: when many traders want leveraged longs, futures trade above spot and the basis widens.
Learn more: spot vs futures.