A preview gives you a look at something that hasn't been released yet. You see lots of previews at the movies.
If you've been to a movie lately, chances are you saw plenty of previews — little highlights of upcoming movies that are designed to make you want to see them. Any little snippet designed to get you excited about something to come can be called a preview. If you're writing a novel, you could give someone a preview by reading them a few lines. And you're lucky enough to see a show before it opens to the public, you're previewing it.