What does this mean......? Showing :)?

As long as your polo looks nice and would be something you could wear to church or a dressy/casual place and not stick out like a sore thumb, you should be fine. At jumper shows they're typically not very strict on dress. Make sure you tuck your shirt in and wearing a small belt probably wouldn't be a bad idea. Jumpers in schooling shows are almost always very casual, but hunters are judged on how they look so they're more strict. A quarter zip polo would probably fine as long as it looks nice. Ask your trainer or some people who have been to that show to see.