layouts in wordpress theme. What is that ?

Hello!

I’v just downloaded underscores blank wordpress theme to develop a new theme and found that there’s some folder called ‘layouts’ and 2 files in it: content-sidebar.css and sidebar-content.css

I’v started looking in google and then in wp documentstion what for does these files stends and found no answer!

Could you explane what for they are needed and how can i use

/*
Theme Name: themename
Layout: Sidebar-Content
*/

in my themes ?

Thanks.