Okay, so you know how your computer has different parts that work together to make it run? One of those parts is called DOS, which is like the boss of your computer's software.
Now, sometimes your computer needs to use a special mode called "protected mode" to make sure all the different programs running on your computer don't interfere with each other.
When your computer is in protected mode, it needs a way to talk to DOS and ask for things like help running certain programs or using specific resources. That's where the "DOS protected mode interface" comes in! It's like a special telephone line that lets your computer talk to DOS while in protected mode.
So whenever your computer needs something from DOS while in protected mode, it just picks up the phone and uses the DOS protected mode interface to ask for help!