Keyword 'layout' in controller rails 5

Hello everyone, when I run the command “rails new my-app --webpack = react” and rails generate react_on_rails: install … when i open the hello_world_controller.rb file, i see keyword layout " hello_word " . And when I create a second component, what is the layout “…” in the controller … I do not understand how it works … please give me an idea … thanks a thousand times

The layout in the hello_world_controller.rb is telling that controller to use the hello_world.html.erb file in the view/layouts folder.