On this screen you can change archive templates for Blog Posts and Portfolio Posts. To find a right page, that you want to assign as layout, start typing a Page Title in the corresponding field.
How you can use this option
By defaults, page with Blog Posts displays in One Column Style, to change this, assign any page with any of available Blog Shortcode in content to Blog Archive, Blog Category and Blog Tag templates, and change shortcode settings to “2 columns style”, for example. Now all blog posts, categories and tags will be displayed using Shortcode Settings Style from assigned page.