The gameplay is divided into "story mode" and "free roam".
The story mode, as the name implies, consists in progressing through the story, getting to know the main girls and unveiling the mysteries of the island.
Most of this in VN-like fashion. You can always leave the story for later and focus on doing whatever you want.
Flirting with the girls, Obtaining new items, customization(eventually) and even finding new monster girls through the island. It's your call.