I too was in this situation once: my first Custodian badge was awarded for reviewing a suggested edit when I had just over 200 reputation.
This is possible if someone with under 2,000 reputation edits a post of yours, you get notified to review it (note: users with 2,000 or more reputation can bypass this as they do not need their edits reviewed). This happens regardless of how much reputation you have, and it's nice that you as the author of the question are allowed to review such edits in case they're invalid.
I believe this is the only review type a user can participate in (albeit in a very limited way) without first requiring a certain amount of reputation.