Heart Rate Reserve is, simply, the difference between your Resting Heart Rate (RHR) and your Max Heart Rate (Max HR).
It is used to be able to determine the Heart Rate zones for an individual and is more accurate than simply using Max Heart Rate since it can be more personalised to a person's specific athletic ability.
Lionheart uses the industry standard Karvonen formula to determine the HRR:
HRR (Heart rate reserve) = Max HR - RHR