When you reply directly to a person, Twitter begins the tweet with an @ symbol and the user name. For example, "@CoolestGuyEver is the bro-est bro around". Twitter assumes that this is personal conversation and it will only push that tweet to other users who follow you and CoolestGuyEver.
Twitter does this so that our feeds aren't inundated with personal conversations. If you have started using Twitter then you can probably already see how helpful it is to not have everyone's "bro-est bro around" conversations showing up in your feed.
Sometimes, however, people begin a tweet with a username but it is something they want all of their followers to see. To keep Twitter from thinking it is a personal reply, you need to put something in front of the @ symbol (you can put anything, the period is just typical). For example, ".@CoolestGuyEver is having a birthday bash tomorrow." will be sent to all of one's followers.