Usually google is extremelly useful :) Also, I believe the Manor Manager gives an explanation x)
Basically it is like spoiling. You get seeds from the manor manager (when used on a mob - like spoil), and from the same npc you get a harvester (it's a scythe that you use on a "spoiled" mob's dead body to get the planted fruit).
Bring these fruit to a manor manager and sellect to deposit them. There will be info about the fruit (what kind of reward you can get for it). You can sellect to which castle to sell the fruit (different castles give different ammounts of materials). When depositing fruit - you can do this at any Manor Manager. When you buy seeds - you have to buy them at the town around which you level. Also in the describtion of the seeds it says what level you must be to use this seed (if lvl too high - sweeping will give 0 fruit usually, if too low - very hard to spoil). Also the same level on the seed describes what level of monsters you must seed.