The new feature associates each window with a profile and lets you sync some of the settings with a Google account. Until now, you could create new profiles manually, but you had to use special shortcuts for each profile (Chrome has briefly added support for multiple profiles in the interface back in 2009, but it was quickly dropped). This feature is now available in the interface, even though it's more limited and you can only identify profiles using Google accounts.
"The multiple profiles feature will allow the user to associate a profile with a specific set of browser windows, rather than with an entire running instance of Chrome. Allowing different windows to run as different Chrome identities means that a user can have different open windows associated with different Google accounts, and correspondingly different sets of preferences, apps, bookmarks, and so on -- all those elements which are bound to a specific user's identity. Having multiple profiles in the Chrome browser also makes it easy to browse with separate identities without having to log in as separate users at the operating system level," explains a Google Chrome design document.