PlotPoints is an option for plotting functions that specifies how many sample points to use.
The sample points are equally spaced.
In Plot, an adaptive procedure is used to choose more sample points.
With a single variable, PlotPoints -> n specifies the total number of sample points to use.
With two variables, PlotPoints -> n specifies that n points should be used in both and directions.
PlotPoints -> , specifies different numbers of sample points for the and directions.
See The Mathematica Book: Section 1.9.3 and Section 1.9.3.
See also: PlotDivision.