Water-wet rock holds water on the grains and tends to imbibe water. Oil-wet rock does the opposite, holding oil on the grains and leaving water in the middle of the pores. Many reservoirs are mixed, wetting differently from pore to pore or even across a single grain.
Wettability changes residual oil, the shape of the relative-permeability curves, and whether a waterflood sweeps well. It is measured on preserved core. Cleaning a plug with solvents often makes it water-wet, so a curve from a cleaned plug can describe a rock the reservoir is not.