Well, my husband giggles like an 8-year-old boy every time I say "dryer balls", but I still think they're fun.
The point of dryer balls is to help make your laundry (in this case- diapers) dry faster and fluff them up to soften them. They don't help with static like dryer sheets do... but since you're not supposed to use dryer sheets with cloth diapers, these are a fun alternative!
You can buy them lightly scented, which will actually make your laundry smell nice. The scent doesn't last a very long time, but you can re-scent them with essential oils or fragrance oils.
People use 2-9 balls in their dryers at a time. And yes, they *DO* make a thumping sound while drying!!! ;)