When you click send, the email is sent to the other person's email account over the internet. They'll get your email the next time they log into their account. If the email didn't reach the other person, you'll usually get an automated email telling you that the email was undeliverable. This could be because the email address was spelled incorrectly, you accidently included spaces, or sometimes it's because the other person's inbox is full and can't receive any more emails until they delete some emails to make space for new ones.