What is exposure? There are 2 parts involved in exposing your camer’s digital sensor to light. The first is the intensity of the light and the second is the period of time the light is exposed to the sensor.
The calculation is Exposure= intensity x time.

The f stop or aperture is the iris in the lens that allows a measured amount of light to reach the sensor. This f stop is determined mathematically by the size of the iris opening of the lens, the lenses focal length, and the dimensions of the sensor. Each of the f-stops either doubles or halves the amount of light allowed through the lens. Camera lenses these days are calibrated into thirds of a stop, such as 4, 4.5, 5, 5.6, 6.3, 7.1, 8. This divides each stop into thirds. F4 means that basically the hole on the lens is ¼ the length of the lens. F8 means the hole in the lens is 1/8 the length of the lens. An f-stop is a factor and the divisor of a fraction.
One of the ambiguous parts of f-stops is the larger the number the smaller the opening. For instance, a hole that is 1/8th the length of the lens is smaller than a hole that is ¼ the length of the lens.
The other part of an exposure is the amount of time that the film is exposed. The longer the shutter is open, the longer the light has to reach the light sensor, the shorter the shutter is open, the less time there is to expose the sensor. The lens measures and focuses the light the camera opens and closes the shutter and stores the image.
Today’s camera shutters are now calibrated in thirds of a stop so you have shutter speeds like 1/30, 1/40, 1/50, 1/60, 1/80, 1/100, and 1/125th of a second. Now the reason both f stops and shutter speeds are broken down into thirds of a stop is to allow for a more accurate exposure.
There is a relationship between f-stops and shutter speeds. Each full f-stop either halves or doubles the amount of light entering the camera and each full shutter speed stop either halves or doubles the amount of time of the exposure. Modern cameras automatically do this for you. It is possible to have the same exposure with a variety of different f stops and shutter speeds depending on what effect you want to achieve. In aperture priority, changing the f stop automatically changes the shutter speed for a proper exposure; conversly, in shutter speed priority, changing the shutter speed will automatically change the f stop for a proper exposure. You don’t have to manually change both factors of an exposure with today’s cameras. New photographers have a hard time understanding the relationship.
If the camera automatically does automatically, why should you care what your f stop or shutter speed is? Shutter speeds stop action or blur it depending on how fast or slow of a speed you use. Aperture controls depth of field (how much of an area in your photo is in focus). You use this to isolate subjects with a soft background or create images that are perfectly sharp near and far. Knowledge of depth of field (f-stops) and shutter speed give you much more control over your camera. Letting the camera decide is like rolling the dice, you may or may not get what you want.
To stop action you have to use a fast shutter speed and lose some depth of field. Each time you use a faster shutter speed, which cuts the length of time the film is exposed to light, you need to open up the aperture to let more light in to get a proper exposure. If you want everything in your photograph to be sharp you use a small aperture (high number). In these situations, you may nee4d to use a tripod because the slower shutter speed is needed for a proper exposure. This is because little light is being allowed to pass through the lens you need to slow down the shutter speed to allow the sensor in your DLSR to be exposed for a longer time period.