I found a great website the other day. A seasoned commercial airline pilot seems to be blogging from inside the flight deck. Some great photos of what he sees out the window also accompany his text.
One of his entries talks about a take off problem. Because of the recent weather (a wet runway), the weight and balance calculations had to be changed. Weight and balance calculations are used to determine if the plane will be able to take off, land, fly properly, etc.. It takes into account the centre of gravity (you can change the COG by shifting fuel from one wing to another, or moving cargo around) and the actual weight of what you are carrying.
For this particular flight, they were able to calculate that 10 passengers were needed to leave the plane, in order for the flight to be able to take off. I find it amazing that they you are able to cut it that close (what happens if the 10 people were lighter than you expected?)