An example of gradualy sized beads. anal beads come in a size range like this example, or three to five on a string of the same size.
They come in all variety's of material from silicon to plastic to glass.
In males they stimulate the prostate and can produce powerful orgasms and aid in becoming erect for multipile orgasms. there is also undocumented evidence that prostate stimulation may lead ro multipule orgasms for men. I'm not sure what the truth level there is though.
For women they provide a "filled" feeling and the stimulation of the muscles in her anus as they are slowley pulled out.
Here's an example:
