The polarizer lets through light that is vibrating in one direction and absorbs light that is vibrating in all other directions. When the black surface is horizontal, the reflection looks dimmest when you hold the filter so that it lets through just vertically vibrating light. The reflection looks brightest when you hold the filter so that it lets through just horizontally vibrating light.
Light becomes completely polarized parallel to the surface at one particular angle of reflection, called Brewster's angle. Brewster's angle for water is 53 degrees; for glass it is 56 degrees; for plastic the angle varies but, in general, will be somewhere between these two angles. Brewster's angle is traditionally measured from a line that is perpendicular to the surface. To find the angle measured directly from the surface you must subtract Brewster's angle from 90 degrees.