ConfigureServices defines the services used by the application like ASP.NET MVC Core framework, Entity Framework Core, CORS, Logging, MemoryCaching etc.

E.g. 

public void ConfigureServices(IServiceCollection services) {
 // Add framework services.
 services.AddApplicationInsightsTelemetry(Configuration);
 services.AddMvc();
}

Also Read:

Purpose Of Configure Method in Asp.Net core

Cookie in ASP.NET Core

How to use Exception filters in ASP.NET Core MVC?