So I finally got modding running. Now, editing files, how do weights work?

Suppose there's only 1 drop is allowed, and 2 different items can drop.

If I put 90 in one and 10 in the other under "item weight", does it mean I have 90 % chance to drop the first, and 10 % chance to drop the other? Or I am mistaken?

What about in other cases where I see various item weights and some have 2000, another 500, another 400, another 1500, another 100, another 80. How does it work in that case?