Lensfun contains information about lens mounts of cameras and available mounts for lens models. This way, the calling application can offer the user a list with lenses that fit on their camera in order to pick one. Otherwise, you would see all lenses every time. This can be pretty overwhelming.
But if you bought an adapter for another mount system, you would want to see lenses of that mount system listed for your camera. So, you must tell Lensfun somehow that you own the adapter. You can do so by calling lensfun‑add‑adapter.
The easiest way to use it is to call it with no parameters. lensfun‑add‑adapter will then enter an interactive mode. First you have to enter your camera model. lensfun‑add‑adapter will use that to determine the destination mount. In particular, the extended lens lists will show up for every camera with that mount, not just the one you entered. Then, you have to select the mount of the lenses you can now put on your camera, using the adapter. That's it.
You can also use lensfun‑add‑adapter to reset your adapter configuration. Call it with the ‑‑help paremeter to see all functions.