What do u and F in physics mean?

What do u and F in physics mean?

U is the object distance and F is the focal length
Take the camera for example, if you photograph a tree, the distance between the tree and the lens of your camera is the object distance
The distance from the camera lens to the back of the camera is called image distance. The letter of image distance is v
A convex lens can make light parallel to the main optical axis converge at a point on the main optical axis, which is the focal point. The distance from the focal point to the optical center of the convex lens is called the focal length. There is a focal point on both sides of the convex lens, and the two focal lengths on both sides are equal
I don't know how to ask~