The Collection Bar is an integral part of finishing your Level, and one of the Level Elements.
The Bar has 7 Slots for picking items out of the pile during the Level.
If you pick up three of the same item, they will be cleared from the level and from the game.
The Collection Bar allows you to store items while looking for more duplicates.
Filling the Collection Bar entirely will cause you to fail the level, similar to running out of time on the Timer.