The good teams in this case are San Antonio, Boston, Chicago, and Dallas, the teams with the best records in the NBA. Three of these four teams are particularly good on the defensive side of the ball, Chicago ranking first, Boston second, and San Antonio seventh in defensive efficiency. In the half court, these teams are first, second, and fourth in defense.
Against these teams, the Heat have scored under 100 points per 100 possessions, about 10 points worse than their average. So not only are these defenses very good, but they are also having more success against the Heat than against the rest of the league. The Heat offense was supposed to be unstoppable and this is, well, a far cry from that.