To adjust a lens so that it produces the sharpest visual image on a screen, or on a camera film plane etc.
The light from a single object passing through the aperture is converged by a lens to form a cone with the tip forming a single point. If the tip of the cone is precisely on the film plane then the image is ‘in focus’. If the tip of the cone falls either in front or behind the film plane then the object is ‘out of focus’.