Unwanted glare can ruin a good image.
It can happen when excesses of light enter the camera lens and impinges on the camera's sensor.
The resulting image has areas of burnt out highlights, hazy rendition and unsightly artefacts.
In most cases, glare is unwanted and unpredictable.
With a digital camera you can see the results of glare immediately by reviewing your images and then make adjustments accordingly.
Shooting towards or into the sun or other light source is a prime reason for glare.
The easy answer is to move your shooting position so that the sun, or other light source, is to one side or even behind you.
It really is that easy.
The other cause of glare is from reflections - from shiny surfaces such as metal and glass or from water.
In this case you could either change your shooting position or use a polarising filter.
Don't worry about how a polarising filter works, just remember that it is great for reducing reflected light so that your image captures the underlying subject without being affected by stray light.
The beauty with a polarising filter is that it can be adjusted or moved and you can see the resulting glare increase or decrease as you do it.
In this way you can select the position that you feel gives the best effect (glare isn't always bad).
Glare can also be used creatively.
Experiment to find out what you feel works best.