Aperture can explain as the size of the opening of the lense. It can be thought as an "iris" of the size that control the amount of light through adjusting the size of the hole. The larger the size of the opening, more light can be captured by the sensor and vice versa.
Aperture is measured in f-stop. Some examples are f/2, f/4, f/5.6, etc. Adjusting the f-stop by 1 means doubling or halving the amount of light entered into the camera. To compensate it, the shutter speed will also change as well either by doubling or halving. There will not be an overall change in the amount of light entered as either the aperture or shutter speed changes as they will be compensated at the end. Below is an example of how the aperture and shutter speed will change. All pictures are taken at ISO 200.
f/2, 1/320
f/2.8, 1/160
f/4, 1/80
f/5.6, 1/40
f/8, 1/20
Notice as you change the aperture the sharpness will increase. This is also an important factor when you want to get a sharp picture. But an increase in aperture will also decrease the shutter speed. This means that taking at a low shutter speed will risk of image blurring.
Aperture also links to depth of field. Depth of field (DOF) means how much of an object is in focus. From the example above, as you increase the aperture, you can see the amount of object in focus increases which contributes the sharpness of the image. Depth of field is useful when you wan to isolate an object away from the background. This will make an object stands out.
Another point to take note is the relationship between the size and the f/stop which many people are confused. It is common for people to assume that a large aperture means a large opening of the lense which is wrong. The reverse is also true but the truth is a large aperture value means a small opening of the lense. Likewise for a small aperture value means a large opening of the lense.
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