“It works by heating the water molecules in the top 1/64-of-an-inch layer of the skin,” said Marine Corps spokesman Maj. David Andersen.

According to reports, a 2-second burst from the system can heat the skin to a temperature of 130° F. Elsewhere, the AFRL describes the sensation as similar to touching an ordinary light bulb that has been left on for a while. “Unlike a light bulb, however,” says the AFRL fact sheet, “active-denial technology will not cause rapid burning, because of the shallow penetration of the beam and the low levels of energy used.”

Edit (me, not the article): I'm not saying that this information is necessarily true, this is just some info I found that was at the top of the search results, from 22 years ago, that I put no effort into fact-checking with other sources. And I wouldn't be surprised if they're used for crowd control, but I don't know. But the second half of the parent comment seems to be accurate.