They usually will give a reason.

Did you read the review guidelines carefully and adhere to their tenets?

Often the reason is a lack of detail, so pay attention to that.

I just submitted my first review and it came back "unapproved". Does anyone know why this might happen??

...go to the top of the page and click on the "My TER" tab. Then look to the left still at the top and click on "My Reviews". Then look across the top at the headings. On the far right is "status". There should be an explanation there for why TER didn't approve your review. If the reason doesn't make sense then do as mrfisher said and contact TER admin and ask for clarification.

I found where you can the reason why it wasn't accepted. Not enough detail. Thanks for the help.