The AC stands for adult contemporary and the hot part means popular music. It's similar to the CHR format but generally for a bit older audience.
The listeners tend to be female and about 18+ in age. It includes popular music with a more mature sound from the 90s and 2000s with some earlier iconic tracks. These stations tend not to include the hip hop tracks, hard rock, and pure pop you might hear on CHR and may not include dance-influenced music. Some artists you would traditionally hear on these stations include Carrie Underwood, Kelly Clarkson, Maroon 5, Sara Bareilles, Goo Goo Dolls, Bruno Mars, Madonna, Backstreet Boys, and Train. More recently though, some stations have started to lean a bit more in the direction of CHR, so it might also include acts like Katy Perry, Lady Gaga, Rihanna, and Usher depending on the station and their target audience. Songs tend to be played for a longer period of time on Hot AC stations than on CHR, as evidenced by the fact that many of their songs are older hits, and some Top 40 hits will go on to then climb the charts of Adult Contemporary.
What do you think? Do you listen to Hot AC? How would airplay on this format help a musician?