What is the thought experiment that proves the relativity of length? What does the expression

What is the thought experiment that proves the relativity of length? What does the expression “shortening the length of a moving body” mean?

The experiment consists in the fact that the length depends on the frame of reference in which it is measured, that is, it is a relative value. The length turns out to be the largest in the frame in which the body is at rest.
The expression means that bodies moving relative to the observer contract in the direction of their movement. This relativistic effect is called Lorentzian length contraction.

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