Premium index
The premium index measures how far a perpetual futures contract trades above or below the spot price index. It is the part of the funding rate that pushes the perpetual back towards spot.
When the perpetual trades above the spot index, the premium is positive and pushes the funding rate up, so longs pay shorts; below spot, it pushes the rate down. On Binance the funding rate is the premium index plus a fixed interest part (0.01% per 8 hours), kept within ±0.05% of each other, so in a calm market, with a premium near zero, longs pay about 0.01% per 8 hours by default.
Learn more: what is the funding rate, and who pays it? and mark price.