"Enrollment over secure transport" is like when you want to join a club, but the club wants to make sure that you're really you and not someone pretending to be you. So they ask you to fill out a form with your name and important information, and then they check to make sure everything is correct.
But since this is a very important club that does really cool things, they also want to make sure that nobody can see your private information while it's being sent to them. This is where "secure transport" comes in.
Imagine you have a secret message that you want to send to your friend, but you don't want anyone else to see it. So you put the message in a special box that only your friend can open with a special key. That's sort of like how "secure transport" works - it puts your information in a special box that only the club can access, and nobody else can see it.
So when you join the club, you fill out a form with your name, address, and other important info, and then it gets sent to the club in a special box that only they can read. This way, the club knows that you're really you, and your private information is kept safe.