The meteorites they studied are called shergottite meteorites, and they likely broke off from Mars about 2.5 million years ago. Even though the meteorites had a different elemental composition, the amount of water was consistent, bolstering the idea that they're representative of the planet as a whole.
Water under the mantle also clues us in a bit to the planet's geological history, suggesting H2O played a role in its formation.
So how much water are we talking about? At least in the samples, a lot of it. On average, a little more than Earth has. The samples suggested parts of Mars have between 70 and 300 parts per million water, while Earth's mantle averages about 50 to 300 parts per million. Two questions it raises: Could Mars ever have sustained life in the past, and would this make it easier to do in the future?