With the Impersonate functionality a user can work on behalf of another user.
This means that a user account can specify other accounts (user or group) which can operate with their account. In other words, if user-B is allowed to impersonate user-A, then user-B can take actions using the full account details of user-A (including ID, name and access rights).
This allows the impersonator accounts to complete tasks as if they were using the account they are impersonating; for example, during an absence or to share an excessive load short-term.
If an account impersonates another it is very difficult to see. The log files hold no information about the fact that impersonation has occurred on the events. So if user-B is impersonating user-A all events will look as if they were performed by user-A personally.