Summarized, the copycat effect is when you see someone else do something (often stupid or illegal), there's a much much higher chance you'll do it too. Let's say you've been waiting to cross the street at a red light for 5 minutes and you really just want to cross. Then you see the person next to you cross. And then another. And another. You'll most likely eventually follow the flow.
The dangers of this copycat behavior are obvious. Even something such as reporting teen suicide rates would increase teen suicide rates.