The PtOn command draws a point on the graph screen. It uses point coordinates, which means that the result is determined by window variables: the x-coordinate must be between xmin and xmax, and the y-coordinate must be between ymin and ymax (with (xmin,ymin) being the bottom left and (xmax,ymax) the top right corner).
Unlike pixel commands such as PxlOn, however, PtOn won't give an error if the coordinates happen to be outside these bounds — it simply won't have any effect.
PtOn can also be used with two lists of the same size. In that case, it will draw the points for every pair of elements (xlist[n], ylist[n]). This can be used as an alternative to plots (see NewPlot) to plot a set of points.