Twitter is trying to decrease the barrier of entry for its new users, and one of the ways it is doing that is by improving the sign-up process so people get better content in their feed from day one. The social network will no longer simply throw out suggestions of who to follow, it will instead ask new users which categories interest them, and then it will begin telling users who they should follow.
One potential issue with this new sign-up process is that once you click on the categories that interest you, Twitter builds you a customized list. If you go through with that list and do not de-select any of the users on it, you will be following all of the people recommended by Twitter.
The goal of this update is to get new users more active on the network by letting them see all of the worthwhile content that is on it.