A Forecast is an estimate that you make, regarding the revenue amount you can generate in a given Forecast Period (Month / Quarter / Year). It would contain a Forecasted Amount , the amount that you can confidently make, and a Best Case Amount , the total amount that you might be able to make.
As a reference, we provide you with the pipeline opportunity summary report in that same period. We provide both the sum of the full amounts, and the sum of the weighted (amount * win_probability) amounts. These numbers can give you a reference estimate, Best Case (Full) and Forecasted (Weighted) amounts. Your forecast can choose to use or disregard these references.
If you are a Sales Manager, your forecast should contain those of your own, and your direct subordinates.