Could you explain what the transaction isolation level Read Committed means?

The Read Committed transaction isolation level means that a transaction can only read data that has been committed by other transactions. This prevents reading 'dirty' data, which are uncommitted changes that could be rolled back.

In practice, this means that if one transaction modifies data, another transaction will not see these changes until the first transaction commits. However, phenomena like non-repeatable reads can occur, where data read in one transaction may change if another transaction modifies and commits it.

Thus, Read Committed provides a balance between data integrity and performance, and is widely used in databases such as Oracle and SQL Server.