This the the iris size of the shutter, it controls the amount of light allowed to enter the camera.
This is measured in ‘F stops’.
F1.4 for example is a very large hole, F22 is a very small hole. The choice of aperture will effect the ‘depth of field’. It’s the distance between the nearest and furthest field where everything is in focus.
A large aperture setting (F stop) give a larger depth of field, whereas a small number (F stop) gives a shallower depth of field.
Looking through the view finder, allows you to see the depth of field at maximum aperture, this allows you to compose pictures with the brightest view possible. To preview the depth of field, press the depth of field preview button on the bottom left of the lens on the housing. This stops the lens down to show the full depth of field.