moomin OK, let's give this a go. Imagine you're hoovering your living room, and you want to go through and hoover the kitchen. At the moment, you'd just walk through the doorway, wouldn't you? Your husband might be standing there (to make sure you don't miss any spots) but he lets you through.
Now imagine that doorway is a 'hard border'. Every time you want to get through with your hoover, your poor husband has to give you a thorough search, inspect your hoover and fill in several forms. Terrible arrangement, right? You wouldn't be able to get dinner on till gone 8!
NOW imagine that when you got married, your marriage vows included a clause (the backstop) to stop such an inconvenient situation from arising, no matter what.
With me so far? Good.
At the moment, you and your husband are getting divorced (not really, don't panic! But imagine you are). He's understandably a bit cheesed off, and is considering implementing a hard kitchen border just to make sure you don't cause any more havoc on your way through. Well, luckily, you've got the backstop to prevent all that!
We just have to hope now that the EU is as reasonable and obliging as I'm sure your husband is in real life! If not - well, if Boris can't talk some sense into them, I'm afraid nobody can!