Application portfolio optimization is a structured approach to improve ROI from application investments. It involves defining goals, creating an application inventory, assessing the portfolio based on business and technical parameters, and optimizing through actions like retiring, replacing, re-engineering or retaining applications. This helps address issues like lack of business-IT alignment, redundant applications, and inefficient adaptation. APO benefits include strategic cost management, improved processes, governance and efficiency, and improved agility to address business needs.