Saltwater corrodes firefighting equipment and may harm ecosystems, especially those like the chaparral shrublands around Los Angeles that aren’t normally exposed to seawater. Gardeners know that small amounts of salt – added, say, as fertilizer – does not harm plants, but excessive salts can stress and kill plants.
It’s the amount of water to fill an acre sized area with 1 foot of water.
Acre inches and acre feet are used in a lot of land use and water use analyses. If a crop needs a certain number of inches per year of rain, or calculating the depth of flooding a certain amount of rain will cause, or how much water can be diverted from a river while fulfilling obligations to downstream rights owners, etc.
It’s like watt hours or calories or light years or electron volts: not exactly an SI base unit but sometimes an easier unit for certain types of conversions and formulas.