The Scrapbook is a gallery of all the players available to join your list in Mr Football. They start in a darkened state but once you own them they light up. When you complete the player, they could add to your Milestones if they are part of a group of mates.
It is not necessary to complete the Scrapbook with all 600+ player cards in a season to succeed in the game. In fact, you will probably not do this unless you are a particularly hardcore coach. That’s okay! When the season resets, the Scrapbook resets as well apart from Legend cards which carry over to the new season.