Isolation has to be enforced by something, and locks are the classic mechanism: shared for reading, exclusive for writing. Taking locks is not enough on its own, so Two-Phase Locking adds a rule about when you may release them, which guarantees serializable schedules. You meet this when asked whether a schedule is conflict serializable or why strict 2PL avoids cascading rollbacks. Basic 2PL does not prevent deadlocks, which is the next problem to handle.