Water-cement ratio is the ration of mass of water to the mass of cement. Numerically it is around 0.5, depending on the desired strength and workability.
A low ratio results in a dry mix which is difficult to compact, with a risk of developing voids in the final concrete. A high ratio weakens the concrete, and is highly undesirable if strength is important. To improve both the strength and workability of the concrete, sometime plasticizers are added.
It would have been appropriate to post this question in the Engineering section (if one is available).
