Normalizing takes any length of vector and turns it into a "unit" vector, with length 1. This is mostly helpful when calculating things like direction (your case, perfect!), because if you then want to use that direction along with a speed or velocity variable, you don't want the "length" of the direction to interfere with the calculation. For example:
Thanks guys im going to try em all to see what I get...
First just one question from Xaer0 what is that direction.normalize() and how do I use it?
If I click on a point all the way across the screen, the initial vector calculated will be incredible long. That represents both the direction and the distance from the player/vehicle to my clicked point. If I used that vector in a multiplication with my bullet's velocity, it would be magnified by how far away the point was (and therefore, I could fire much faster bullets by clicking far away, and slower ones by clicking near myself). By normalizing it, the distance of that vector is changed to 1, while preserving the "direction" value. Multiplying anything by 1 doesn't change your outcome, so it's safe to use in your bullet velocity and travel case.
Edit: forgot to add, it's also crucial when trying to do other vector math like dot and cross products. Without normalized vectors in those instances, your results will be skewed and/or represent different properties of their related data than what you intended.