Okay, kiddo, when you edit a video, you usually use a computer program to make changes to the footage. Offline editing means you do this editing without being connected to the original, high-quality footage.
Think of it like a coloring book. When you want to color a picture, you usually make a copy of that picture so you can color it without ruining the original. This copy is like the footage you're editing.
Now, let's say you're at a friend's house and you want to color in your book. But you left the copy of the picture at home. So what do you do? You use a different, maybe lower-quality version of the picture to color in your book. This is kind of what happens when you do offline editing.
You use a lower-quality version of the footage to make your edits, then later, when you're all done, you reconnect to the original, high-quality footage to make those changes to the video. Kinda cool, huh?