Do you know what a "continuum" is? It's like a really long line or a spectrum, with different things on one end and different things on the other end. Now, imagine there are a bunch of people who all speak a language called "creole." But, because there are so many people who speak this language, there are different ways they might say things or different words they might use.
Now, imagine that line or spectrum we talked about earlier. One end of the line is where people speak in a very "pure" version of the creole language, with very few differences from one person to another. The other end of the line is where people's speech has changed a lot over time, so much that it's really different from the original creole language.
And guess what? In the middle of that line is where most people speak - their language isn't exactly the original creole language, but it's also not totally different from it. This middle part is called the "post-creole continuum." So basically, it's like there are different versions of the same language, and where someone falls on this continuum depends on how much their speech has changed from the original creole language.