A convex lens is a lens formed by two spherical surfaces bulging outward. It converges a beam of light incident on it and hence is known as a converging lens. A beam of light parallel to the principal axis of the convex lens passes through the lens and converges at a point called the principal focus. The distance between the optical center and one of the focus points is called the focal length of the lens.
The rays of light coming from a distant object such as the sun (or a distant tree or a distant building) can be considered to be parallel to each other. These rays enter the lens and after refraction converge at the focus on the other side of the lens. The real and inverted image of the distant object formed at the focus of the lens can be captured on a screen. Thus, the distance between the lens and the screen corresponds to the focal length of the convex lens. This video explains determination of the focal length of a thin convex lens by obtaining the image of a distant object.