ASP.Net MVC Core: setup view area location path

Question

i read a article which discuss how to setup custom path for view

 

http://www.c-sharpcorner.com/article/expanding-razor-view-location-and-sub-areas-in-asp-net-core/ 

 

one area is not clear regarding routing code setup.

 app.UseMvc(routes =>      

            {
                routes.MapRoute(
                    name: "subAreaRoute",
                    template: "{area:exists}/{subarea:exists}/{controller=Home}/{action=Index}/{id?}");
                routes.MapRoute(
                    name: "areaRoute",
                    template: "{area:exists}/{controller=Home}/{action=Index}/{id?}");
                routes.MapRoute(
                    name: "default",
                    template: "{controller=Home}/{action=Index}/{id?}");

            });

what is the meaning of {area:exists} code ?

what it does ? what it is checking ?

 

please help me to understand this. thanks