The COCOMO model is a software cost estimation model that classifies projects into three categories - organic, semi-detached, and embedded - based on team experience and project complexity. It uses effort, duration, and staffing constant values that vary for each project category to calculate the effort required to build software based on lines of code. The purpose is to help estimate effort, staffing needs, and costs for a project based on its characteristics and the constant values table provided by the COCOMO model.