SQL Server blocking occurs when one connection places a lock on a table (or selected rows, pages, extend) and another connection attempts to read or modify the data when the lock is in effect. Another connection has to wait till the resources are released from the original connection which is holding a lock on resources.

Often blocking happens on the server where the system is under heavy transactional workload on a single resource. The way to resolve the blocking is to identify the statement which is creating blocking and optimize them (re write T-SQL, Indexing, other configuration changes).