MySQL shared lock and exclusive lock

Share lock shared Lock (also called read lock)
Also called a read lock, if the transaction T with the data object a plus s lock, then the transaction T can read a but cannot modify a, the other transaction can only a plus s lock, and cannot add x lock, until T release S lock on A. This ensures that other transactions can read a, but cannot make any modifications to a before T releases the S lock on a. shared locks do not block shared locks, Block exclusive locks

Exclusive lock exclusive Lock (also called writer Lock)
Also known as write lock. If the transaction t has an X lock on the data object A, the transaction T can read a or modify a, and the other transaction cannot add any locks to a, until T releases the lock on A. This ensures that the other transaction cannot read and modify a until the lock on the A is released by T. Exclusive locks block all exclusive locks and shared locks.

Read why read lock: Prevents data from being read by other threads with write locks

MySQL shared lock and exclusive lock