Aperture is adjustable on manual lenses or on a camera body when you’re in manual mode. It opens to let in more light, yet oddly the smaller the aperture number the bigger the opening. If it was getting dark in the afternoon you might set your lens at f2.8 or the lowest number your lens can go. The consequence of opening the aperture to let in more light is your depth of field gets shallower and less of your photograph is in focus.
Photographers will shoot with a wide open aperture not just to let in more light but to creatively isolate the subject, letting everything else go out of focus.