Aperture is the iris inside a lens — a ring of blades that opens wide to let light flood in or closes down to restrict it. It is measured in f-stops, written f/1.8, f/4, f/11 and so on. The counterintuitive part is that smaller numbers mean bigger openings: f/1.8 is a wide aperture, f/16 is a narrow one.
Aperture does two jobs at once, which is why it is the most creatively loaded control on a camera. It sets brightness, and it sets depth of field. Open up to f/1.8 and you get a bright image with a shallow slice of focus and a soft background. Stop down to f/11 and you get a darker image with far more of the scene sharp front to back.
Because those two effects are welded together, choosing an aperture is always a trade. Portrait shooters open up to separate a face from its background. Landscape shooters stop down to hold detail from foreground rock to distant ridge. Fixed-aperture cameras — including most pocket and companion cameras — make this choice once, at the design stage, and optimize everything else around it.