...Ready? Here goes ...
When you @ someone, the only people who see the message are the people that follow you and follow the person you are @-ing.
So, if we write, "@thestalwart you are one smart dude!" The followers who don't follow @thestalwart don't see the message.
Why? Because if they saw us tweeting at people it would be a confusing one-sided conversation. If they follow both of us, then they can follow the conversation.
Now, let's say we want everyone to see the message we are sending to @thestalwart. How do we make that happen? With a "." So, it would be, ".@thestalwart You're a genius!"
You don't have to use a period. You could use any character. You could write, ">@thestalwart you rock!"
The period seems to be the most popular one, though.