When your database is busy with multiple sessions reading and writing data, the mechanism that controls access to data files and memory areas can become a consideration for performance tuning. Otherwise, sessions can spend time waiting for access to resources when they could be running concurrently.
MySQL manages contention for table contents using locking:
Internal locking is performed within the MySQL server itself to manage contention for table contents by multiple threads. This type of locking is internal because it is performed entirely by the server and involves no other programs. See Section 7.10.1, “Internal Locking Methods”.
External locking occurs when the server and other programs lock table files to coordinate among themselves which program can access the tables at which time. See Section 7.10.5, “External Locking”.