Why is there @interface above @implementation?

I am wondering why there is twice @interface. One in class.h and other in class.m. For example:

TestTableViewController.h:

#import <UIKit/UIKit.h>

@interface TestTableViewController : UITableViewController

@end

and (automatically generated) class.m i find:

#import "TestTableViewController.h"

@interface TestTableViewController ()

@end

@implementation TestTableViewController

... methods delegated from UITable delegates

@end

So my question is, what the @interface TestTableViewController () in the .m file is about. Why it is there? Do I need it?

Thanks in advance