They look like this,
dryer balls.
I have four of them. They work like tennis balls, perhaps more effectively because of the little bumps on them. They help separate the clothes in the dryer, keeping them from flopping around in one big lump so they dry more efficiently.
I use all four of mine most of the time and hardly ever use dryer sheets anymore. I have to use sheets for fleece because I live in a very dry area and if I don't the static is terrible. With all my other loads I just use the balls. Low static, soft enough (I was never big on soft, I use the sheets for static), and it cuts about 10 min or so off my drying time.
When I do use a dryer sheet I rip off about 1/4 of a sheet and throw it in with the balls and it works well. So I'm using much less of the sheets than I had been.
Overall they've been a good investment for us.