Imagine that your message is written with letter-play pasta on your table. Now imagine you dump all these letters into a plastic casserole and shake the casserole. It would be already hard enough for someone who doesn't know your message to just reconstruct it with those letters, right? Now imagine that you dump a whole package of additional letters together and shake it all up. At this point, your original message would be encrypted and impossible to reconstruct.
As this is a very complex subject, its detailed technical explanation is by no means the aim of this article.
If you want to know more and explore these concepts, you can start by doing so in the links below: