There's a page that unlocks in the in-game manual after your first game that explains them. Love is primarily used for getting different endings and is gained from conversations and dating. Lust goes up from various interactions and resets from sexual events. Lewdness increases up to a limit from most events. Bond level goes up for each completed quest.
Different scenes will occur based on how high the stats are, the galley gives hints on the requirements for each one.