This is an entry in our Disneyland 101 Glossary: Disneyland Resort definitions in under 101 words, from Mouse Brief.
Some attractions at Disneyland Park and Disney California Adventure Park have a pre-show. A pre-show is an introductory video or other type of entertainment before the main attraction. Sometimes safety guidelines are incorporated into the pre-show, as in the video shown before Indiana Jones Adventure. Other times they serve to give instructions for an interactive ride, like in WEB SLINGERS: A Spider-Man Adventure. In all cases, they take the attraction’s story, previously started with the queue decor, and present it more explicitly. Some attractions, like the recent Mickey & Minnie’s Runaway Railway weave the pre-show fairly seamlessly into the main attraction.
For ways to have fun while you wait in line, check-out Make the Best of Your Time in Line at Disneyland and for a preview of what happens when it is time to board, see What It’s Like to Board a Ride at Disneyland.