"you can't cross the sea merely by standing and staring at the water." what does it mean and why does it matter?

It means that you can't accomplish a big job by just sitting around and doing nothing, and thinking you can't do it. You have to do something about it and not stand around! It matters because it's true haha and I deal with the same thing, I have to do something or I want to do something but it looks too hard so I don't do it and just worry instead. 

This is math not english