At some point, Twitter-ers came up with their own method of directing updates to one another using an @ symbol. We started supporting this behavior by doing things like creating the in reply to link, confining the reply messages to friends in the loop, and linking the @username to a profile. Another layer of support to this behavior is a feature we launched tonight which collects updates directed at you—just in case you missed ‘em.
The Replies page is where you can go to see all the updates that folks have intentionally Twittered in your general direction. In other words, if somebody Twitters “@biz liking the new Replies tab!” it will get saved at Twitter.com/replies. For more info, check out these support documents Crystal wrote up: What is the Replies tab?, and What does @username do?