Great Blue shows the wildness and scattering of the stack - if you get 3 or more Sea Shell symbols (scatter) anywhere on the reels, you will trigger the Great Blue Bonus feature. You will automatically start 8 free spins and 2 multipliers. Then you need to choose 2 of the 5 sea shells and you can win extra free spins and/or multipliers.
In total, you can win 33 free spins and/or up to 15 multipliers. Also, keep in mind that if you get 3 or more scattering during free spin, you will trigger an additional 15 free spins that will play in the same multiples as the trigger spin. Finally, the number of free spins that can be won is unlimited.