A tack coat is a light spray of asphalt emulsion on an existing or milled surface before a new lift of hot mix goes down.
Its job is bond. Without it the new lift sits on the old surface as a separate layer, traffic shears the two apart, and you get slippage cracks and delamination within a few seasons.
Rates are given as residual asphalt, meaning what's left after the water in the emulsion evaporates. FHWA's 2016 tack coat guidance recommends about 0.02 to 0.05 gallons per square yard on new asphalt, 0.04 to 0.07 on existing asphalt, and 0.04 to 0.08 on a milled surface. Emulsion is only part asphalt, so the rate you spray is higher. If the bill of lading shows about 60 percent asphalt, 0.05 residual means spraying about 0.08 gallons of emulsion per square yard. Your spec governs if it says otherwise.
Let the emulsion break, turning from brown to black, before the paver runs over it. Tack is cheap next to the lift and easy to skip when the distributor truck is late, so put it on the proposal and the foreman's work order as its own line.