What is blocking in SQL Server?

Blocking occurs when one session must wait because another session holds an incompatible lock on a resource it needs.

Some blocking is normal in transactional systems. It becomes problematic when transactions are long-running, queries touch more data than necessary, indexes are poor, users leave transactions open, or application code performs slow work while holding database transactions.

Troubleshooting should identify both the blocked session and the head blocker rather than focusing only on the waiting query.