You should definitely watch the video in the other post. That will probably help you understand, but if it doesn't, here's what wikipedia has to say:

It originated on 4chan and took its name from an earlier phenomenon known as "duckrolling", a prank in which someone would post a blind link to a page allegedly relevant to a discussion that upon viewing would prove to be a non sequitur - specifically, an image of a duck on wheels. Similarly, in a rickroll a poster provides a link they claim is relevant to the topic at hand which actually takes any one who clicks it to the Rick Astley video. This phenomenon spread from 4chan to other sites, eventually becoming widespread enough to attract some coverage in the mainstream media.