Aperture
Aperture is the adjustable opening within the camera lens. The opening is adjusted with aperture blades which widens or narrows the hole.
The smaller the aperture value(f-number) is the wider the aperture is. This is because the f-number is the denomenator: therefore the smaller the value is the bigger the opening
The wider the aperture, the more light it lets in; thus the image would be brighter.
Wider aperture also blurs the background more, due to shallower depth of field