The aperture is in the lens of the camera and controls how much light comes in. The wider the aperture, the larger the space where the light comes in. This is called shallow depth of field. The smaller the aperture, the tinier the space where the light comes in. This is called deep depth of field. Aperture looks like f2, f16, f20, etc. on your camera. The smaller the number, the wider the aperture (i.e. f1.4), and opposite for the tiny aperture (i.e. f32).
shallow depth of field (f1.4) deep depth of field (f22)