An f-stop is a way of measuring how open the camera's aperture is. Aperture works like the pupil in our eyes; becoming wider in dark situations to allow more light in, and smaller when in bright spaces to reduce the amount of light. F-stops can be opened or narrowed in increments of third, half and full stops, ranging from f1.2 (wide open) up to f22 (very narrow).
An f-stop also determines the photograph's depth of field. A low f-stop of 1.2 will create a very blurred background, whereas an f-stop of 22 will produce a very sharp photo across the entire frame.