A query plan details how the SQL Query Optimizer will run the query. The optimizer plans which portion of the query to execute first and which index to use. Depending on the way the tables are set up, the nature of the query, the indexes available, etc. SQL Server will determine the optimal technique - be it to scan through the table from start to finish, to use one or more specific indexes or various other ways.
SET SHOWPLAN_TEXT ON – TSQL, Include Actual Execution Plan - SSMS
The query plan is stored in the SQL Server memory’s Procedure Cache.
