Swallowing goldfish is the act of placing a live goldfish into your alcoholic beverage of choice, and then slamming the drink, goldfish and all. Usually performed by particularly drunk college students at frat parties.
I would guess the goldfish dies not too long after being swallowed, from the combination of the alcoholic drink itself, and from the gastric juices that are secreted shortly thereafter.
On a personal note, my friend Tom swallowed a goldfish once, and the one word he used to describe it before promptly passing out was "Wiggly".