Definition
An isolation level is the contract describing which anomalies concurrent transactions may observe. Weaker levels permit more anomalies and cost less coordination. Read committed prevents dirty reads but allows a value to change between two reads in the same transaction. Repeatable read and snapshot isolation stabilize what a transaction sees, but still allow write skew, where two transactions each read a consistent snapshot and then make writes that are jointly invalid. Serializable forbids everything by guaranteeing some serial order exists.
The practical point is that defaults vary and they are rarely the strongest option: PostgreSQL defaults to read committed, MySQL's InnoDB to repeatable read. Code written assuming serializable behavior on a read-committed database is a well-known source of bugs that only appear under load — the kind that pass every test and corrupt data in production.
How it comes up in interviews
This shows up whenever a design has a check-then-act step: verifying a seat is free and then booking it, or checking a balance and then debiting it. Interviewers want to hear that the check and the act must be atomic — via serializable isolation, an explicit lock, or a uniqueness constraint that lets the database reject the double booking for you.