Without Longboard bearings, you're board won't go anywhere.
Longboard bearings are essentially the little wheels built into your wheels. They are the point of contact that lets your wheels rotate while the truck stays in place and are rated using the ABEC bearing ratings system. You can learn about that here.
One big difference between longboard and regular skateboard bearings is a built-in spacer. The best longboard bearings come with a spacer built-in. This keeps the side-load forces that happen on hard carves and slides from warping and crushing the bearings, so your setup runs smoother for longer.
You'll generally want new bearings, or to at least clean your bearings, if your board is rolling slowly or getting very noisy. This is caused by increased friction or damaged bearings. If they are damaged you'll want to buy new ones, if not you can learn how to clean your bearings here.