The light bulb produces unpolarized light - each photon is vibrating in its own different direction. Nonmetallic surfaces, such as black plastic, tend to reflect light that is vibrating parallel to the surface and to transmit or absorb light vibrating in all other directions. If the black plastic is horizontal, then it reflects light that is vibrating horizontally, creating horizontally polarized light. The horizontal black plastic reflects less light that is vibrating vertically.
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.
Horizontal surfaces in the environment, such as the asphalt of a street or the surface of a lake, reflect light that is vibrating horizontally. Polarizing sunglasses absorb this horizontally oriented glare. If you tilt your head sideways, this horizontally oriented glare passes through the glasses, making the surface look brighter.
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.