Water Bound in Martian Soil
A fine-grained Rocknest soil sample analyzed by the Curiosity rover in Gale Crater contained about 2% water by weight. NASA says the water molecules were bound to amorphous materials in the particles. Converting that fraction using a representative bulk soil density gives the popular approximation of about two U.S. pints of water per cubic foot.
This does not mean every cubic foot of the Martian surface contains the same amount of readily extractable liquid water. The result refers to a particular sample and water released during heating/analysis; soil water content varies by location and composition, and the water is bound to the material.