Hi, sorry first time trying to publish something, getting used to the platform. It is fixed now.

Hi,

First of all you can solve this Kata with a simple loop that check whether the letter is uppercase (in which case you would add a space before it).
To your question, these are Regular expressions. Plainly speaking it is an easy way to find and manipulate patterns in a string.
The [A-Z] means any upper case character and $1 means the first group you found (so they add a space and $1).

Here is a website you could learn about the subject: https://www.vogella.com/tutorials/JavaRegularExpressions/article.html