An American slang term for a hot dog, frankfurter, or wiener. Traditionally used by roaming hot dog* vendors, especially at baseball games; "They're red hot! Get your red-hots here!".
Aside from the fact that red-hot emphasizes their well-heated state, the term was most likely reinforced by the red food colouring added to many brands of hot dog (some hot dogs are red 'naturally' -- but you'll pay extra for that).
'Red-hots' is most often seen in the plural. Almost every dictionary will hyphenate it; in popular usage it is almost never hyphenated.
* the term 'red-hot' probably predates the term 'hot dog,' which came into use around 1890. I have been unable to find a date for the origin of the term 'red-hot', but it is older.
Red-hot, either hyphenated or not, can be used as an all-around intensifier (That's a red-hot car!) or exclamation (red-hot, man!). It means good, great, cool, and excellent. It may also refer to something that is extreme or outrageous.
It can also refer to the state of someone who is sexually aroused, as an intensifier to 'hot'.