There are different kinds. One kind is a metal ring, another is an elastic strap, another (Lazy Hands) has straps for each finger. They only cost a few bucks.
I've tried both the strap and the ring, they are both really good. (The ring is good quality for the money.) Both of them comes off most materials without too much trouble or residue, despite sitting securely.
There are other types too, such as Popsockets. I don't recommend that one though, they are over-priced and mine came apart when I tried to move it.
With the ring especially, I find that I can walk around with the phone in my hand continually without strain or fear of dropping it. That way I have the camera ready, and I can quickly skip back in an audiobook without having to dig it out of a pocket.
And it makes an even greater difference with an ereader or tablet, because you can't get your fingers all around those. Holding a tablet is dramatically easier.
|Me with the Ring Holder on an iPad Mini.|
Notice it is hanging only on my finger here!