Aperture
Aperture units are in the letter ' F '. If you are taking a picture in a place where there is not a lot of light then the lower the F number to allow more light in. If you want less light you would use a high F number for example on a sunny day. I have learnt that when I am taking photos outside I have to keep adjusting my aperture depending if a cloud goes over the sun or if I am shooting into shadow. The aperture is also linked to depth of field.