Heat refers to both crowd reaction and real-life animosity between those involved in the professional wrestling business. In terms of crowd reaction, heat is usually either cheers for a babyface or boos for a heel. The amount of heat a wrestler generates is often an accurate gauge of his popularity.
Although the term can in some contexts refer to either positive or negative crowd reactions, "heat" can otherwise be used specifically to mean a negative crowd response (booing etc.); its opposite being a "pop" or positive reaction (cheering etc.).