An air cannon, as shown above, is a tube like device with a plastic wrap at the end. There's a knob on this plastic wrap that one pulls back as tight as possibly. When the knob is released, the plastic wrap is pushed forward, and shoves a volume of air out the tube in a column. At the Science Day, cups where set up in a pyramid pattern to allow for a bit of target practice with the air cannon. My daughters needed some help pulling the knob back tight, but once they got the hang of it, they were all about knocking those cups down.
You can get one yourself fairly easily. They are cheap to buy. You can get several off of Amazon.com for around $15 - 20
There are several websites with instructions on how to build your own. Here's one:
Building one is a great activity for you and your child, but even if you buy one already built, you've got an afternoon of fun with your kid(s)!