Quadratic equations

Hi all, I’m trying to learn how to solve quadratic equations but I’ve ran into a problem.
On the wikipedia page: http://en.wikipedia.org/wiki/Quadratic_equation
It explains the step by step process of solving a quadratic equation using the “Complete the square” method. The fourth step states: “Write the left side as a square, and simplify the right side, if necessary.”
And this appears to change the expression “x² + 2x + 1” into “(x + 1)²” and I’m not exactly sure how that’s come about. Could somebody explain it to me?

Thanks!

Paul