Repair Outer Defenses is a project in Civilization VI. It becomes available after building Walls. A city can undertake this project if it and/or its Encampment district have damaged Walls and have not been attacked in the last three turns.
Walls gain HP equal to the Production invested into the project (on Standard speed) each turn the project runs. Once completed, it fully restores the HP of the city's (and Encampment's) Outer Defenses.