Shows the number of sessions that are waiting on locks held by others.
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Blocking in SQL Server occurs when a session that was performing some task is unable to progress because it must wait on a resource that is currently being used by another session. The resource that is being waited on can be either a physical structure like a lock on a table or an internal SQL Server resource such as a latch.

The SQL Activity drilldown | Blocking page shows details about the current blocks.

See also the The SQL Activity drilldown | Deadlocks page.