A Slippery Slope argument generally attempts to argue against one action "A" because of the belief that it will inevitably lead to a much worse effect "B." An example of this might be:
If you allow homosexuals to be married (A), the next thing you know they're going to have to let people marry other things like animals and trees (B)!
In this case the one presenting the argument skips over all of the intermediary steps necessary to get from "A" to "B." The conclusion in "slippery slope" arguments might be an actual concern of the arguer, but that doesn't mean we should assume "B" will follow from "A" especially if there are no obvious causes and effects linking "A" and "B" together.
In order to avoid this gap, the arguer may lay out their own string of causes and effects that lead from "A" to "B." Here's an example that might bridge the gap:
1. Allowing homosexuals to get married will destroy the traditional definition of marriage.
3. If marriage is opened to everyone and everything, then people will be able to marry animals and trees!
Even though an argument might be successful in imagining a system of causes and effects in which "A" might eventually lead to "B" that doesn't mean the system is correct. There is no evidence that allowing gay marriage would actually lead to open marriage to anything. Nor is it even possible for plants or animals to enter into legally binding contracts or to sign a marriage license.
The general idea to understand about a "slippery slope" argument is that the arguer wants to make "A" look a lot worse than it would standing on its own and therefore discourage people supporting that proposition. Often times the claim that "B" will happen is unsupported by any actual evidence.