A vene dash admin user is given the access to "Login as" button, inside the publisher details under the tab "Publisher Access".
What does it do?
It gives you the ability to do a "Shadow Login", meaning you can log in into the account of your publisher without having an additional account. By this, you can for example support your publisher in case he needs support on his side or if you want to support with his setup.
Click on "Login as", enter your password and confirm. You will be redirected to the publisher interface of this specific publisher.