To catch an edge is the most annoyingly painful thing that occurs when you are learning to snowboard.
What happens is this: you have been learning the basics for a few hours (or a few days if you are a little less lucky) and you are starting to get the hang of it. You know how to turn, how to stop, and how to slow down. You are, in your own mind, unstoppable. So you start to go fast
On this descent, you begin to feel so confident that you let up your razor-sharp concentration for a second. It's only a second, but that's all it takes. You catch an edge.
On the most basic level, what you did was let the leading edge of your board touch the ground when you didn't want it to. This has a similar effect to when a pole vaulter lowers his/her pole. The edge digs in to the snow, and you are pitched either forwards or backwards, depending on if you caught the heelside or toeside edge. If you caught your toeside, you're lucky. You can easily catch yourself. Heelside, you fall right on your ass. Hard. A bruised ass is not uncommon during the learning stages.