By default, the problem "weight" is equal to the amount of points the problem is worth. If the problem is made up of multiple parts, each with their own submit button, then those parts behave as if they were separate problems, and each one has its own weight.
If you want the change the default, you may want to think of a problem's weight in proportion to other problems. For example, if all problems have a weight of 1,changethen both easy and difficult problems have the same weight. If you choose to change the weight, then the problem weight and points may not equal each other for your course.
The default weight of all problems and problem parts is 1.