After losing your crew on Sabaody Archipelago and then your brother in the Summit War, you came to the realization that you just weren't strong enough to continue your pirate career. Silvers Rayleigh offered to train you for two whole years about Haki, so now you're on the island Rusukaina.

Haki is the spiritual energy in all living things. It represents your will power. Not many can bring it out, let alone master it. Even two years barely scratches the surface. Nonetheless, I'll teach you what I can during this time.