In a nutshell, what is a rash on the scrotum in men?

Scrotal rash. Rashes on the scrotum can be from a number of causes. Commonly yeast grow in the skin creases and cause jock itch. A variety of other conditions including psoriasis, warts, heat rash, chaffing can also cause these and you should see a urologist or dermatologist to accurately diagnse and treat these.
Lots of things. Interesting way to pose the question! unless it has been there for years, this could be an infection or could even indicate something like diabetes. Needs an examination to find out. Keep it dry, wear loose shorts, could try otc "jock itch" preparations.