So what it does and how to use it?

So i thought it's like used to make the vector larger to increase the width or height of it but i don't know how to use it, and if that's what it does then i need it very bad.

The length of a vector can be used in various ways. You should check a source about vector properties and operations somewhere in order to find out what the length is used for.

http://en.wikipedia.org/wiki/Euclidean_vector

As for the length formular it is: length = Math.sqrt(x² + y²).