The Bonus Symbols are part of the Bonus Pool feature in Lucky Slots.
Sometimes, when you spin, a bonus symbol shows up on the reels. It gets added to the bonus collection on the left side of the screen, starting a 12-hour timer for you to collect all five bonus symbols for that machine. As you continue to bet and spin, the bonus pool counter goes up. If you collect all five symbols within the time limit, you get a payout equal to the bonus pool counter multiplied by a number between 2 and 10. If time runs out before all five are collected, the bonus pool symbols and amount reset to zero.