In crochet, the word "frog" has a couple different definitions.
Frogging Crochet Stitches
Frog can be used as a verb. In this sense, "to frog" means "to rip out stitches."
When used this way, the word is slang, and it is also a play on words. It pays tribute to our amphibious friends, the frogs, and their choruses of "ribbit, ribbit, ribbit". When you discover a mistake in your crochet work, you rip it, rip it, rip it – hence, you frog it.
Knitters and cross stitchers also use this term in the same context.
Frog Buttons for Crocheted (or Other) Garments
Frog can also be used as a noun. When used this way, it indicates a fancy closure for a garment. Frogs can enhance dresses, sweaters, vests, tops, shirts, robes, and other articles of clothing.
Like buttons, frogs can be either functional or decorative. They typically include 2 parts, one attached to each side of the garment. One part functions as the button, the other part functions as an external button loop.
Frogs often feature some sort of knotwork. For example, many frogs incorporate Chinese knots.
Frogs can be made using fabric, cord, gimp, thread, yarn, string, fiber, or similar material. You could even crochet your own frogs. (See this crocheted frog closure tutorial, which includes photos.)
I had to frog an entire sleeve because it didn't match its mate.
I just went on a frogging spree; I frogged all the UFOs I've had hanging around for more than a year.
My sister's new cheongsam has intricate frog closures.