The Basal Metabolic Rate (BMR) tool is designed to estimate the number of calories your body needs to maintain basic functions while resting. These are functions like breathing, maintaining circulation, cell production, and other metabolic processes. It can serve as your starting point for the minimum calories you need to carry on with your current status.
These are only estimates and should not be taken as the end-all, be-all. Talk with your provider about steps you can take to get back on track if you aren't already.