The Set Active Record ID for Screen action will give a value to the Screen Active Record ID.
Every time you log into Method, you are entering a new session, which is how Method keeps track of information like the current date and time, who's logged in, and the record ID of the record you may be accessing.
Example: A method user selects a contact from the contacts grid. They are taken to the View Contact screen which shows the contact they selected. Behind the scenes, when a user selects a contact on the grid, the Record ID of the selected contact is placed in the Screen Active Record ID. When the View Contact screen loads, it will use the Screen Active Record ID to find the contact in the Contacts table and load the values onto the screen.
When the Screen Active Record ID is set, fields on the screen will be populated with values from the record who's record ID is stored in the Screen Active Record ID.