What does it mean to say water is 11% hydrogen by mass

It means that if you get any mass of water, hydrogen will compose 11% of it.

Explanation:

It is due to the fact that water is mainly made from H2O molecules and the hydrogen and oxygen atoms have their own mass too. So if you get any mass of water and divide the mass of hydrogen in that amount of water by the total mass of water you'll have a fraction of around 0,11, which represents the relation between the quantity of hydrogen and water.