Decoupling Asp.Net Identity 2.0 from Entity Framework in MVC5

read this article https://jinishbhardwaj.wordpress.com/2014/07/16/decoupling-asp-net-identity-2-0-from-entity-framework-in-mvc5-part-1/

but see the below code which is not clear

public class ApplicationDbContext: DbContext
{
    static ApplicationDbContext()
    {
        Database.SetInitializer<ApplicationDbContext>(null);
    }
    public ApplicationDbContext()
        : base("DefaultConnection")
    {
        this.Configuration.ValidateOnSaveEnabled = true;
        this.Configuration.AutoDetectChangesEnabled = true;
    }
    public new IDbSet<T> Set<T>() where T : class
    {
        return base.Set<T>();
    }
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Configurations.AddFromAssembly(typeof(IdentityModel.Configurations.IdentityUserConfiguration).Assembly);
    }
}

1) what is the below code.......is it indexer ? how it will be used and what it will do ?

public new IDbSet<T> Set<T>() where T : class
    {
        return base.Set<T>();
    }

see this line

protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Configurations.AddFromAssembly(typeof(IdentityModel.Configurations.IdentityUserConfiguration).Assembly);
    }

modelBuilder.Configurations.AddFromAssembly(typeof(IdentityModel.Configurations.IdentityUserConfiguration).Assembly);

what the above line is doing ? please guide me.

thanks