Every site has its own independent user log in and permissions model. This provides a very powerful and flexible structure, however for a site designer or IT department providing a support role, it can be essential to login and see what is happening.
Login impersonation allows the owner of the parent site to use their login to gain access to the child site, without the embarrassment and security risk of having to ask the user for their site login credentials.
To login as your client, use your username (as the Owner of the parent site) with an asterisk (*) in front of your email address, ie:
Use the password you normally use to login as the Owner of your own site.