Isolation level defines what data an SQL transaction can view or access while other transactions work with the same data.
ACID standard defines 4 isolation levels - read uncommitted, read committed, repeatable read and serializable. The higher the isolation level is, the more is guaranteed that another transaction can't break yours, but the lower amount of concurrency the database can handle. MySQL and MSSQL support all isolation levels, while PostgreSQL and Oracle support only the 2 most common, read committed and serializable
Read uncommitted means that the transaction works with the latest data available. In this isolation level it's possible that a transaction reads data that is not yet committed and possibly will be rolled back and never exist.
Read committed is the most basic isolation level, which ensures that transactions only read data that is already saved. It is possible however for another transaction to modify the data after the first transaction has read it but before it has modified it.
Repeatable read ensures that any subsequent read of the data will return the same result as the first read, therefore eliminating the race condition described above.
Serializable ensures that transactions are run in such a way that the result is the same as they were run in sequence, not in parallel.
