Is the time, your MacBook should used on battery every day. It is the base of all recommendations from Better Battery and it's calculated by your activity data of the last 30 days.
Lithium-ion batteries work in charge cycles. You complete one charge cycle when you’ve used (discharged) an amount that equals 100% of your battery’s capacity — but not necessarily all from one charge. For instance, you might use 75% of your battery’s capacity one day, then recharge it fully overnight. If you use 25% the next day, you will have discharged a total of 100%, and the two days will add up to one charge cycle. It could take several days to complete a cycle. The capacity of any type of battery will diminish after a certain amount of recharging. With lithium-ion batteries, the capacity diminishes slightly with each complete charge cycle.
A battery's capacity is the amount of electric charge it can deliver at the rated voltage. The more electrode material contained in the cell the greater its capacity. A small cell has less capacity than a larger cell with the same chemistry, although they develop the same open-circuit voltage. Capacity is measured in Miliamperehours (mAh).
The capacity of a brand new battery when leaving the factory. It depends on model and size of the battery. For example, a MacBook batterys factory capacity is greater than the capacity of an iPhone.
The capactiy of a battery diminishes with each charge cycle. So the Battery health is showing the current maximum capacity compared with the factory capacity in percent.