The collection are 5 lucky icons: The Horseshoe, A four leaf clover, A Pot of Gold, Fuzzy Dice, and a Rainbow.
Collection pieces are placed on your game board when you earn a boost from the bingo boost. You need to successfully daub that square in order to capture the icon. If you get bingo before you capture the icon you do not receive that icon.
When you collect all 5 Collection Icons you earn 500 GVs! You will see them on your Game Over Rewards screen.