Glare is an unwelcome word when discussing photography lighting. Whether it is flash reflecting brightly into the camera lens or a reflection of a light somewhere else in the room or a glare caused by the bright sun or some other light source, glare is something photographers rarely want to see. Although glare can occur with any reflective surface, it is most common with glass. Windows, glasses, and other objects made of glass reflect light very well so they can cause a lot of glare. In order to avoid glare you need to know how light reflects off glass.
Light is linear
While it is possible for light to bend around objects under certain circumstances, light generally behaves in a linear fashion. This means that your school teachers really were right. You will use math in real life, although in this case you don't have to be precise with your figures. Unless modified, light reflects at an angle equal but opposite to the angle at which it strikes the glass.
This means that if you shine a light (like a flash) directly at a piece of glass it will reflect directly back into the camera and produce glare. Even if you diffuse the light striking the glass the reflection can still be a problem.
Next - Light at an Angle and Avoiding Glare