Prefix headers are compiled and stored in a cache, and then automatically included in every file during compilation. This can speed up compilation, and lets you include a file without adding an import statement to every file using it.
Example:
#ifdef __OBJC__
#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h>
#import "Constants.h"
#endif
Add it in your NameOfMyProject-prefix.pch file.