Ball bearings make skateboards (and anything else with wheels)go faster because they reduce the friction between the wheels and their axles. In general >when things slide over each other, the friction between them causes a force that tends to slow them down. But if the two surfaces can roll over each other (which they can if there are ball bearings between them), the friction is greatly reduced.

There is an excellent web-site called "How Bearings work" which shows a movie of a skateboard wheel rotating around its axle with the ball bearings in between:

Check it out! Bearing balls.

If the ball bearings were not there, the wheel would scrape directly over the axle. Not only would this slow things down, but eventually the wheel or axle would wear out and break.