How does the Gig delivery Review work

[design4you1]

Hi guys,I’m new to fiverr, this might be a really stupid question but I’m just wondering about when you deliver your gig to your buyer and it says Buyer:please review. What does this mean? The count down still goes down also, so what happens if the buyer doesn’t review it before the timer goes to 0. I would like to hear your thoughts, Thanks