Every professional bar and even some home bars have a bottle of triple sec behind it, but what the hell is it? Triple sec is made from dried peel of oranges that are found on Curacao, an island in the Caribbean. It's strong (usually contains 30% alcohol/60 proof). It's a sweetening/flavored agent used in a variety of mixed drinks.
"Sec" in French means dry. Dry then indicates a lack of sweetness. This doesn't apply though when you are talking about triple sec. In the case of triple sec, it just means triple distilled.
Most of the time it's also colorless unless if it's blue curacao which is basically blue-colored triple sec. Anytime you hear the term "blue" used in the name of a drink or see that the actual color of a drink is blue, it has blue curacao in it.
Triple sec can come in fancier forms such as Cointreau and/or Grand Marnier. These are considered "top shelf" triple secs (aka: more expensive). Grand Marnier has a slightly yellow/light brown tint to it because it is a cognac-based triple sec. How is that for fancy?
Here is a list of some popular drinks that have triple sec in the recipe:
Adios Motherfuckers (AMF)
Margaritas (Cadillac and Top Shelf)