A leash is a rope or similar material - such as a metal chain - used to restrain or control an animal.
Bajorans used leashes to control rim runners. (DS9 novel: The Pet)
The phrase on a short or tight leash was used to indicate a person was lacking in independence or autonomy due to being strongly controlled by another. After discovering Captain Jean-Luc Picard had assisted Section 31 in the removal of President Min Zife from office, Admiral Leonard James Akaar advised Picard that he was on a tight leash, and that Akaar was holding the other end of the leash. (TNG novel: Hearts and Minds)