Isolation levels decide how much of a concurrent transaction's changes become visible to another. A non-repeatable read is when reading the same row twice returns two different values because another transaction committed an update in between. A phantom read is when running the same range condition twice returns a different set of matching rows because another transaction inserted or deleted rows in between. PostgreSQL maps Read Uncommitted to Read Committed. For example, if a price is updated from 100 to 120, Read Committed lets a second read within the same transaction see the new 120, while Repeatable Read uses a stable snapshot instead, keeps seeing 100, and prevents both non-repeatable and phantom reads. A serialization anomaly — a result that could not happen under any one-at-a-time ordering of the transactions, even though each looks correct alone — can still occur. Serializable also detects serialization anomalies, so applications must retry on failure.