From a software performance standpoint, "hot spots" are an area of intense activity. They're hot spots because they're frequently executed code paths with some sort of friction or bottleneck. They represent potential optimization paths for improving the performance of your code or design. You find the hot spots by measuring and analyzing the system.
Stepping back, we can use "Hot Spots" more loosely. We can use them to gather, organize, and share principles, patterns, and practices for performance.