Isolation mode

Read uncommitted aka dirty read allows reading data that have not been committed by transaction. 

Read committed allows only retrieving rows that has been committed. But a repeating read may return different result as there are committed changes in between successive read

Repeatable read allows the same result to return in successive read but it still allows phantom read. Rows that inserted between successive read. T only protected the rows read from changing. 

Serializaruon is to do one query at a time thus preventing all changes until the current transaction commits.