Adapter (adapter ring) is a device that enables the use of camera and lens combinations from otherwise incompatible systems.
Using special adapters (adapters) allows you to set the lens to the camera of another manufacturer.
In addition, it saves you from buying the expensive optics of the corresponding system.
For example, there are adapters that allow you to set the old Soviet lenses (Zenitar, Helios and so on) on a modern camera.
So you can use a very high quality lens at a very good price.