The Layout tab by default displays the same information that is configured for proper Data Source Object (see Salesforce help). If user wants to show something other, he can define necessary step layout in step properties at Process Builder.

Default layout is Standard in View mode (It is actually the Form with fields defined in Object config):

User can set mode to Edit - in this case users can be able to modify some record fields:

User can set step layout to any other suitable form in View mode:

...or Edit mode:

User can select a Visualforce page as layout and it will be displayed (and will treat parameters):

Also, user can specify a Lightning Component to be displayed as layout (FormPanel in this example):