It is water rate divided by water plus oil, usually in percent. Gas is left out. A water cut of 80 percent means four barrels of water for each barrel of oil. Water-oil ratio is the same information written as a ratio, water over oil, rather than as a fraction of the liquids.
A rising water cut in a waterflood is the flood arriving, or water channelling through a high-permeability streak. A rising water cut in a well near the water-oil contact can be coning. The two have different remedies. The cut is a surface measurement of the liquid; it is not the water saturation in the reservoir.