Target Heart Rate is utilized to determine the intensity at which a person should be exercising based on the goal for that session. It is fairly common for people to monitor their heart rates during a session of physical activity but a lot of times they do not know what to do with their numbers.
According to Polar, one of the leading manufacturers of heart rate monitors, the most utilized key target zones are 60-70%, 70-80% and above 80% of a max predicted heart rate. The 60-70% range is for a person that wants to lose weight and build endurance. The 70-80% range is for weight management and improving cardio fitness. And above 80% is for high intensity interval training.
Exercise intensities should be chosen accordingly and based on an individual's limitations. For this reason, it is always recommended to work with a qualified personal trainer that will create individualized plans and respect your limitations to safely progress towards your goals.
There was a time in which max heart rate was calculated by simply subtracting your age from 220. However, a group of researchers led by Tanaka conducted a meta-analysis and developed a regression equation that strongly correlates max heart rate with age. According to their research the (220 - age) equation underestimates max heart rate in older adults.
Tanaka Method:
Max Heart Rate (MHR) = 208 - (0.7 * age)
Knowing an individual's max heart rate, we are able to utilize a method developed by Karvonen to calculate target heart rates. This method takes into consideration the resting heart rate when calculating exercise intensity ranges.
Karvonen Method:
Target Heart Rate = (MHR - resting HR) * intensity + resting HR
The intensity should be computated as decimal. So if a person wants to find the target heart rate for 70%, then 0.7 should be used in the formula.
Polar USA has an article on their website focused on target heart rate zones that can be found here.
Tanaka's max heart rate article can be found on ScienceDirect.