A relearn remote key is not actually an object. It refers to the process when you loose your key (usually to a car), and you have to make the vehicle accept a new key.
You have to reprogram the car so that the second key is now responsible for the automatic locking and in some cases the remote start (although very few cars have this feature).
How to 'Relearn' a remote key widely depends on what car it is for.
In order to find out how to reprogram your car's keys, firstly identify the car and then either contact the cars maker or search online for instructions on how to reprogram the key specifically for the car.
However for all cars, make sure that all doors are closed and that the car has its breaks on.
To show you what I mean, this video shows you how to 'relearn' the remote key for a 2007 Honda. While this may not be exactly how it would work with your specific vehicle - but it gives you the basics of what relearning a remote key entails.