Weight is a combination of complexity and availability. When you give something more weight, you decide that it has more value than something else.
For example, if you have an objective that can be assigned a value, consider how often this objective can be collected, how many instances of this objective are available, and how hard it is to collect. Something that is easy to obtain would have a lower weight, while something that is harder to get, or less frequently available, would get a higher weight.