In an RDBMS, Execution plan is the set of steps that are executed in a specific order to get the results of a query.
Execution plan is used mainly for performance improvement of queries. It helps developers and DBAs to see the internal steps that take place during a query execution.
Based on the steps in Execution plan, we can alter the plan to include or exclude some steps or use some index etc. This helps in Query tuning and Query optimization for SQL queries.