The slippery slope fallacy is when you are in a discussion with someone and they say something like, “If we allow gay people the right to legally marry then PEOPLE WILL BE MARRYING COWS AND LAWN FURNITURE AND DOGS AND CATS WILL BE OWNING PROPERTY AND THE ECONOMY WILL CRASH BECAUSE NO ONE WILL BUY CARS BECAUSE DOGS AND CATS AND COWS WILL WALK EVERYWHERE AND POOP ON THINGS!”
In other words, a slippery slope argument is when someone takes a single situation or idea and in very few steps, blows it way out of proportion in an attempt to prove their point. It assumes an extremely unlikely or impossible cascading chain of events or a single, highly unlikely outcome from the topic being discussed. The conversation goes from rational to irrational in a single swooping-slide like a free falling penguin off an icy bank into the ocean.
It is a slippery slope, my friends and you should do your best to keep your feet planted in reality.
Now, here is a drawing I did of a penguin.