A Query Execution Plan is a detailed roadmap that a database management system, like SQL Server or PostgreSQL, uses to execute a database query. It outlines the steps the system will take to retrieve the requested data, including how tables will be accessed, which indexes will be used, and the order of operations. This plan helps optimize performance by identifying the most efficient way to execute the query. Understanding the execution plan is crucial for database administrators and developers, as it can reveal potential bottlenecks or inefficiencies in the query. By analyzing the plan, they can make informed decisions to improve query performance, such as adding indexes or rewriting queries for better efficiency.