config.ru is a Rack configuration file (ru stands for “rackup”). Rack provides a minimal interface between web servers that support Ruby and Ruby frameworks. It’s like a Ruby implementation of a CGI which offers a standard protocol for web servers to execute programs.
Rack’s run command here means for requests to the server, make Sinatra::Application the execution context from which Sinatra’s DSL could be used. All DSL methods on the main are then delegated to this class.
So in this config.ru file, first you require your app code which uses Sinatra’s DSL then run the Sinatra framework. In the context of Sinatra::Application if your app.rb contained this:
get "https://stackoverflow.com/" do
'Hello world!'
end
The get block would mean something to Rack, in this case when someone tries to access (GET) the home url, send back 'Hello world!'