Serializable provides the highest transaction isolation. When a transaction is on the serializable level, a query sees only data committed before the transaction began and never see either dirty data or concurrent transaction changes committed during transaction execution. So, this level emulates serial transaction execution, as if transactions would be executed one after another, serially, rather than concurrently.
If a row returned by query while executing a UPDATE (or DELETE or SELECT FOR UPDATE) statement is being updated by a concurrent uncommitted transaction then the second transaction that tries to update this row will wait for the other transaction to commit or rollback. In the case of rollback, the waiting transaction can proceed to change the row. In the case of a concurrent transaction commit, a serializable transaction will be rolled back with the message
ERROR: Can't serialize access due to concurrent update
because a serializable transaction cannot modify rows changed
by other transactions after the serializable transaction began.
Note: Note that results of execution of SELECT or INSERT (with a query) will not be affected by concurrent transactions.