To magnify something is to enlarge it. A calling is a divine assignment that you've been asked to do. To magnify a calling is to fulfill your divine assignment to the best of your ability. It's doing the best you can everyday to be a little better at what you've been asked to do. You enlarge your task so you can see the details and begin to perfect them.
It is possible to magnify a calling, no matter how insignificant you may feel you are for the assignment. The Lord doesn't ask us to do things that He knows we can't do. Anything you are asked to do, you can do it. So why not magnify it, or do it the best that you can? Why not serve others through this divine task? You are capable of making a difference, especially through your calling.