To understand how aperture works on a camera it helps to compare it to the pupil of the human eye. The less light there is, the wider you open your pupil, while if there is a lot of light it narrows down to a small opening and blocks the amount of incoming light.
The Aperture Scale of a lens is divided into what we call f stops. By opening the aperture by 1 stop we are doubling the amount of light it lets through. By closing the aperture by one stop we are halving the amount of light.
Most cameras will allow us to adjust the aperture in smaller increments than one f/stop, usually in 3rd stop increments.