If you're antsy, you're restless and fidgety. It's hard not to be antsy when you're waiting to learn whether or not you've been accepted to your first choice of colleges.
An antsy kid, unable to sit still, might need to use the bathroom, and a worried parent is often antsy while she waits for her teenager to come home late at night. Antsy came first, in the 1830s, but the idea is similar to the 20th century expression "have ants in your pants." Imagine how you'd squirm if you actually did feel ants crawling around in your pants: figuratively, that's how it feels to be antsy.