The water-cement ratio (w/c) is the weight of water divided by the weight of cement in a concrete mix. It is the single most important factor controlling the strength and durability of concrete: for a given cement, lower water-cement ratios give stronger, denser, more durable concrete.
Cement needs only about 0.25–0.30 of its weight in water to fully hydrate, but a little more is added for workability — so practical ratios run from about 0.40 to 0.55. Every extra litre of water beyond what hydration needs eventually evaporates and leaves tiny pores, weakening the concrete and making it more permeable. This is why adding water on site to make concrete easier to place — a common bad habit — quietly destroys its strength.
IS 456 sets maximum water-cement ratios by exposure condition: harsher environments demand lower ratios for durability. For typical M20 house concrete a w/c around 0.50 is common. Workability is better improved with admixtures than with extra water.