Rat shows are a fantastic way to meet other ratty people, to find interesting rat goodies to buy, and to show everyone how wonderful your rats are. Check the rules first to make sure you can take rats along as 'shoulder rats' (taken as companions rather than to show), but if you can that is a wonderful way to meet people as they will come up and talk to your rats and break the ice that way. Before you consider taking your rats to a show, they need to be healthy and without any infection or parasites, and to have had no contact with other rats of unknown health during the past two weeks. It's very easy for infection to spread at a show, especially among shoulder rats, and although it helps to build the communal immunity it's a good idea to keep the risks as low as possible.
Most clubs have a 'how to show' section on their website so you can see how their shows work. The UK clubs tend to have two sections, pets and varieties. Both of these sections are judged on their health and condition, with the pet section also being judged on the rats' pet qualities such as tractability and outright lovability, and the varieties being judged on their conformity to the published standards for their variety. I'd recommend entering the pets section first while you learn the ropes, and moving on to varieties as you gain experience.
The prizes at most shows are rosettes, although some shows will have cups to win too. Don't hang the rosettes on your rat carrier unless you want them redesigned.