The String.split(String s) splits the string into an array of strings in places where there is s in the original string. So you have something like "hello world!", and if you do "hello world!".split(" ") it will split the string into an array like this:{"hello", "world!"}
Then with the [0] he is accessing the first element of the array. 0 is the first element in Java and you access array elements by an index with square brackets, remember!

because it refers to step. range(start, stop, step)