As the liquid level rises in the tank, the float is lifted up due to buoyancy. The float carries a stem attached to it and the other side of the stem has a magnet fixed to it. The float stem moves inside an SS enclosure. External to this enclosure is a glass tube, which carries the  magnetic follower (red in colour). Due to  magnetic attraction, the magnet on the float stem and the magnetic follower are coupled and move in unison. So as the float moves along with the liquid, the follower also moves inside the glass tube, and the position of the follower can be read against the scale. (Ref to the front view of the scale enclosure).