An Execution Plan is a visual map showing how SQL Server executed your query. It shows you exactly which indexes were used, how many rows were processed, and where the "cost" is. If you can't read an execution plan, you can't tune a database.
Look for the Key Lookup operator. It means your index isn't "Covering" the query. Also watch out for Sort/Hash Spills (yellow warning icons)—this means SQL Server ran out of RAM and had to use the slow TempDB to finish the task.
Q: "What is the difference between an Estimated and an Actual execution plan?"
Architect Answer: "The **Estimated Plan** is generated by the Optimizer based on statistics *before* any code runs. It is just a guess. The **Actual Plan** is generated *after* the query finishes and shows the real number of rows and memory used. You should always use the Actual Plan when debugging, as the Optimizer can be wrong if your statistics are out of date."