If you set the aperture too small for the conditions that you are shooting in, your photograph will appear too dark (under exposure),and if the aperture is too large, your photograph will appear too bright (over exposure).
Most cameras, particularly SLRs, allow you to set the exposure, and will measure the light available and advise you if the exposure is too great or too little.
Aperture can be used in combination with shutter speed to maintain a constant amount of light entering. If you are using a large aperture (low f-stop numbers) you can decrease the shutter speed to allow the same amount of light onto the sensor. This type
of setting will increase your depth of field – more of your image will be in focus. If you are using a small aperture, you can increase the shutter speed, which will decrease your depth of field, allowing you create the effect where only a small portion
of your image is in focus, and the rest of image maintains an attractive blur.