The health of your wall. Depending on context, “wall health” may refer to several aspects in the game:
The Wall Health lab increases the maximum health of your wall. This lab is unlocked when you unlock Wall in your workshop.
The regular Wall Health workshop consists of 1800 levels with a starting percentage of 20% which increases by 0.1% per level for a total of 200%.
The Wall Health workshop enhancement applies a multiplier to your existing Wall Health.