There are two currencies in Zoo. One is Zoo Dollars, which you will accumulate easily from by clicking the bag daily. One bag click collects all the money for your zoo, so you do not have to go to each island and collect.
Other activities (mystery box collection, tree shaking, visiting other ZooMates' zoos) also render more Zoo Dollars for your zoo. The second is Wildlife Points which allow you to buy premium items. These are accumulated from completing in-game achievements, mastering breeding levels, and completing trophies. You can also purchase WLP through the Wildlife Fund tab.