SQL Server ensures that your data is protected even as the data is changed. It does this through a process called locking. However, even as SQL Server is locking within the I/O system in support of your data changes, this is causing what is called blocking, through contention for those resources. As the number of users on your systems increases, performance degrades as they contend for those resources. In order to understand how to deal with blocking, you need to understand how, and why, locking occurs.