Apertures units are in the letter “F”. If you are taking a picture in a place where there isnt a lot of light then the lower the F number the more light you get in. If you want less light in you choose a higher number. Basically doing the opposite. You would choose a higher F number for example if you were to take a landscape photo, as there is already alot of light, you wouldn't need more.
When you select a fast shutter speed, the aperture will change to a higher F number and when you select a slower shutter speed, the aperture will change to a smaller F number