How important is the hydrometer?

[boo boo]

MY question is how do you know what the gravity is supposed to be? The whole hydrometer thing confuses me.

Even when you know that the OG is suposed to be lets say 40 and it isn't...What do you do? I am trying to understand all the nuts and bolts about this whole thing..haha even though all that matters is it tastes good

Using your OG you then look to the attenuation of the yeast you selected to brew with. The attenuation will tell you how much percentage will be fermented.
If your og was 1040 and your attenuation is 75% then your fg should be 3/4 of your OG or 30 points. your FG will be 1010 give or take ideal fermentation and the amount of unfermentables making up your wort composition.