Fusion Level is one of the first things unlocked in your game. This provides alot of stats to your pokémon, and the max fusion level is +40.
Fusion can be leveled up in two ways: with Fusion Exp Cards or with Fusing Pokémon.
Fusion Exp Cards cab be obtained in the Safari:
Keep in mind that you will lose all the pokémon that you are fusing! So be very careful with this.
When your Character Level is higher, you'll also unlock a new and harder Safari, where you can obtain Advanced Fusion Cards.
These Advanced Fusion Cards will provide you much more experience than the Fusion Exp Cards. This new Safari will also provide you items that you'll need once your Pokémon reaches Fusion Level +40.
You can learn more about this Hard Safari in this video: