Explanation
Forecasting is a feature that allows you to predict and plan the sales cycle from pipeline to closed sales, and manage sales expectations throughout your organization. Opportunity stages are the steps that an opportunity goes through as it moves from creation to close, and they determine the probability and forecast category of the opportunity. References:
https://help.salesforce.com/s/articleView?id=sf.forecasting3_overview.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.customize_opptystages.htm&type=5