A forecast is based on the rollup of a set of opportunities.

A forecast is a projection of how much revenue you can generate in a quarter. A forecast rollup is the aggregation of a set of opportunities based on three dimensions: time, categories, and territories. Time refers to the fiscal period, such as month or quarter, that the forecast covers. Categories refer to the stages of the sales process, such as commit, best case, or pipeline. Territories refer to the sales regions or markets that the forecast applies to. References:

Certification - Sales Representative - Trailhead

Cert Prep: Salesforce Certified Sales Representative - Trailhead

[Forecasting Guide - Salesforce Help]