Salting the earth refers to the practice of spreading salt on fields to make them incapable of being used for crop-growing. This is typically used at the end of a war as an area denial measure.
Perhaps the most famous example of salting the earth occurred at the end of the Third Punic War in 146 BC between the Romans and Carthage. After sacking the city of Carthage and forcing the few survivors into slavery, an area 50 mile around the city was salted. However it was later learned that this event was fictitious: the land was merely cursed, with salt possibly sown in a symbolic furrow around the city.
Today the term is used in a variety of ways, referring in general to any sort of poisoning. This varies from the direct in the use of area denial or radiological weapons, to the philosophical, where it is often used to describe business strategy to avoid takeovers (similar but broader in scope than a poison pill).