Nationals Question

[NotaNerd]

So... here's a long story short. We're a second year team and already on tier 2 status. Don't ask how because I don't know. Anyways, we called up FIRST to confirm this and they told us yes. But that's not the question. When we asked her about registering for nationals, she said that they are already filled up. How the heck can you already be filled up when WE DON'T EVEN KNOW WHAT THE PROBLEM IS? She said that the best thing we could do would be to be put on a waiting list because lots of teams usually drop out. So what happens then if a team wins its regionals? Do they say, "Tough luck, we're already full"? If someone could explain this to me, it would be excellent. Also, is there a difference between "going" to nationals and "competing" in nationals?