Scooter Bearings are a must have for your custom pro scooter. Although mostly unseen, the purpose of the scooter bearing is to keep the friction low between moving parts. You'll hear the term 'play' often when talking about scooter bearings. The less play (or less slop) the better when it comes to pro scooters and the bearings.
If you want a more interesting and technical explanation of scooter bearings, or bearings in general, check out this cool article on wikipedia.