Power - how many watts can you put out
Weight - how much you weigh

You can have a lot of power, think like a sprinter or a time trialist. But if you have a lot of weight that power doesn't help you when you add in the effects of gravity when you climb.

I like this calculator for figuring out my watts. I use the data from a climb that I do often

http://www.kreuzotter.de/english/espeed.htm

V.