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An impersonator is someone who imitates or copies the behavior or actions of another.
So, an impersonation can be understood as imitating behaviors or actions. In the context of ReadonlyREST: one user could imitate an action of another user. Why would we want it? Let's suppose the first user is an admin, who has just configured access for a new user. They would like to know if the rule(s) are configured correctly. And here comes the impersonation feature. The admin can impersonate a given user in Kibana and see what the user would see if they logged in themselves.