The module allows site administrators to switch users (with their ol and permissions) and surf the site as that user (no password required).
This is helpful for site developers when trying to determine what client, logged in as themselves, might see and do when logged into the site.
Install and enable the module.
Create a new role.
Edit the permissions for that role p.e. add content.
Create a user (p.e. tester) and give him/her the role you first create.
Go to the new user y select 'Masquerade as'.
Test the permissions.
With the link 'Unmasquerade' you can switch back to the role as administrator.