Forecasting is about estimating how quickly your team completes tasks and advances. Forecasting ultimately helps you plan more realistically and ensures that as a team you set realistic goals, for instance during the Sprint Planning meeting.
The Scrum framework by the way, talks about frequently inspecting progress. The focus here is on transparency, inspection and adaptation. This means that you regularly check how things are going and make adjustments if necessary. The Scrum Theory, Scrum Values and Scrum Events assist you in this.