This line uses what's called a foreach construction. The user declares step variable and calls a method that returns a collection. The foreach construction will then traverse the returned elements one at a time, using the step variable to represent the current element as it goes through the loop's statements.
You could say the same thing with a normal for loop. This form of expression is simply more concise.
Declaring the step variable final means the Document reference cannot be changed inside the loop. Some people think it could boost performance, but if it doesn't it does no harm. Personally I find the usage a bit obnoxious.
Make visible what, without you, might perhaps never have been seen. - Robert Bresson