What happens when i enable Advanced Permissions?

Hi there
I need to change the site for a client that is already live to have Advanced Permissions.
Does anyone know what exactly happens when i click that? Does the site remain the same and be visible and useable still to guests?

I am worried that i will have to go through a lot of pages manually to fix the permissions if it does the opposite!

Thanks in advance

nothing will happen to the site all the permission will stay the same but you will have extra permission based features. This is not reversible