Tell what you are about in the very first slide of the presentation. VCs like to put in you a box. If you don't do it for them, they will spend the time guessing while you deliver your carefully crafted buildup of the story. People who are busy guessing, do not register other content.
Telling what you are about is not the same as running the entire presentation on the first slide, and repeat the same story on the other 20 slides in the deck. "We are an online book store". "Aah, now I know". No more explanation needed.