Nothing, really. This is just the top row of keys on an English-language keyboard. Tomlinson was just testing out the system and using a nonsense message. He had no concept that he was going to start a revolution in communication.
Tomlinson was one of a group of scientists who were working on developing better computers. The scientists at his site were able to send a message to a "mailbox" on the computer on their site. Other scientists could view the message in the mailbox. But there were other computers at other sites where scientists were working on the same project. Tomlinson's idea was to figure out a way to deliver message to mailboxes on those remote computers. He used the @ sign to identify message that were headed out of the local machine to the more distant ones. That was the start of the emailing systems that we still use today.