This slider adjusts the rest length of the pendulum cord.
The number underneath the slider gives the actual distance of the centre of the pendulum bob from the point from which it hangs.
This will differ from the rest length of the pendulum cord either if the cord is loose or if it is  noticably stretched (by a large gravitational pull or a large pendulum bob mass).
As far as the behaviour of the pendulum is concerned, it is this actual length that is relevant.

investigations