moonlighting in any specialty is when you have the option during your training to work under your provisional license, if you are not above the 80 hour week, to fill in or take call for extra pay. the pay is uaually good, in psych it is around $1000 for a 24 hr shift at the places that i have heard about, but that will vary. the FREIDA website gives some info on moonlighting at each hosp., but beware, some places allow it, but cannot call it moonlighting due to certain regulations, and might call it something like extra call. BU is an example of this, they say on FREIDA that moonlighting is not allowed, but is is, just called something else.
usually you can only moonlight in your 3rd year, some places maybe second...not sure on that. many many people do it, especially in 3rd and fourth year when call is light or not at all. a really great way to supplement income, and even double it.