The price for a single load, right now, at whatever the market is paying today.
Spot is the overflow market. It moves fast, both ways, and small carriers live in it more than large ones do.
Spot rewards you when capacity is tight and punishes you when it is not. That volatility is the argument for having some contract freight underneath it.
Related
Inteleri works out what a load actually pays you once things like this land. See how it works.