The 'plum tea visual novel' may center around the significance of plum tea in a fictional world. Maybe in this visual novel, plum tea has some magical properties. It could be used to heal the sick, or it might be a key ingredient in a potion. The characters in the novel might be on a quest to find the best plum tea or to protect the secret of the plum tea from those who would use it for evil purposes. The visual aspect would likely bring to life the different scenes involving plum tea, from the orchards where the plums are picked to the kitchens where the tea is brewed.
I'm not sure specifically what 'plum tea visual novel' is about as there could be many different stories within it. It might be a visual novel that has elements related to plum tea, perhaps set in a place where plum tea is significant, like a traditional Japanese or Chinese setting where plum tea is a part of the local culture.