What does Test Explorer actually start?

I'd like to run some tests on a web page front end I made to a ASP.NET RESTful Web API, by using Selenium. I'm assuming right now that when I click "Run All" in the test explorer, that it's starting my web server, loading my web page (by going to it's url using a Selenium driver), and then accessing it. I'm not sure what "Run All" actually does though, and would like a little more information. What does it do?