Okay, so imagine you have a big book with stories about a man named Jesus and all the things he did. There are lots of copies of this book, but they don't all say exactly the same thing. Some have different words or sentences, and it can be hard to know which one is the "right" one.
That's where the Alexandrian text-type comes in. It's a group of copies of the Jesus book that come from a specific place called Alexandria. People who study these books have noticed that they tend to have similar words and sentences, so they think they might be the closest to what the original Jesus book said.
It's like if you had a bunch of copies of a picture of a dinosaur, and some of them had extra spikes or different colors, but then you found a group of copies that all looked the same. You might think that those were the most accurate versions of the original picture.
So, the Alexandrian text-type is a group of copies of the Jesus book that people think might be the closest to what the original book said.