Suiton or water release, is an elemental chakra nature that involves the manipulation of already existing water or by converting chakra into water. This water can be used in a variety of ways such as creating a transparent shield, creating large masses of water to hit you opponent and even to douse the field to give some lightning jutsu an extra boost of speed.
Suiton is naturally strong against katon, or fire release, as it extinguishes the fire where as suiton is weak against Doton, or earth release, as the sheer mass of the earth is enough to resit and hold back the forces of the water.