I'd say it's more like teaching classes. I find it helpful to be knowledgeable about the types of toys you're helping to review so you can point out any incorrect information the review might have. Basically what you're doing is reading over a review and giving pointers on how to improve that review. What could be added in to make it better, what could be edited out, what details should be included, etc. Editing for grammar is part of it, but not the main thing you're working with someone on. It's mainly helping to make the review as useful as it can be.
Edit: Also you may want to sign up to take a class yourself if you're considering mentoring. That way you can get a feel for how it works. Even if you don't need the help for your reviews it will help you understand the mentor program more thoroughly.