A bright spot in a dark field is hard for your eyes to adjust to and focus. That difficulty is perceived as glare. You do not want to look at glare. In some cases the glare is so severe that you cannot look at it. In other cases where the glare is mild you can look at it, but the constant muscle adaption can lead to eye strain. Glare is often categorized as distracting, discomforting or disabling.
Glare can be caused by an emitting source of the brightness, like the sun. Glare can also be caused by a reflective source, such as a computer screen. In the case of a reflection the angle of the brightness source to the reflective surface often act as a concentrator for the brightness. There are many things you can do to reduce or eliminate glare requiring only minor changes to the light source, the reflective surface or your eyes.