BitLocker is the Windows encryption technology that protects your data from unauthorized access by encrypting your drive and requiring one or more factors of authentication before it will unlock it.
Windows will require a BitLocker recovery key when it detects a possible unauthorized attempt to access the data. BitLocker recovery key is a unique 48-digit numerical password that can be used to unlock your system if BitLocker is otherwise unable to confirm for certain that the attempt to access the system drive is authorized.