Hot coat is another way of saying sanding coat. It fills in the weave of the cloth, and any imperfections in the laminate.
It's resin mixed with a filler -- the best I've found is West System # 410 Microlight Fairing Compound mixed with 105/205 resin.
Mix it to mayonaise consistency and squeege it on to the final (sanded) layer of glass or carbon.
Use masking tape on the tips of the board when you're hot-coating the bottom of the board to create a dam so that you can sand in a hard edge. Naturaly, keep the other rails soft -- just the tips need a hard edge.
Home board builders sometimes use Bondo for the sanding coat, but it's a lot more work to apply and much harder to sand.