You would ideally like your database application to scale linearly with the number of database users and the volume of data. However, it is common to find that performance degrades as the number of users increases and as the volume of data grows. One cause for degradation, especially associated with ever-increasing scale, is blocking. In fact, database blocking is usually one of the biggest enemies of scalability for database applications.
The fundamentals of blocking in SQL Server
The ACID properties of ...