A bottle of coke is full of carbon dioxide gas (the bubbles.) The bubbles stay suspended in the liquid until the bottle is opened.
When you drop any sort of object into a bottle fizzy liquid, bubbles form on the surface of the object.
The Mentos drops to the bottom of the bottle, forming lots and lots of bubbles on its pitted surface along the way.
When all of this gas is released it forces the liquid up and out of the bottle in a giant whooshing geyser!