It is like a key to start your car, except you can register it with multiple cars. And it has 25 or so "slots" for car registrations. If you lose the key, you cannot order a copy from the car manufacturer. You also cannot make a copy yourself. But you can (usually) register multiple keys with the same car. You do this by plugging two keys into the same car, and the car learns both keys belong to the same owner. You just have to be careful and keep track of which car is registered with which key, and vice-versa. Sometimes the key will not work with a particular car. Also, after you plug in the key, the car will not start right away. It will first ask you to select which key to use. If you use Bitwarden, it may hijack the key insertion interaction and will offer to use its soft-key instead. So, some small differences ;-)