you will get a seed when you start the game and 3 more when you complete your firsts quest.
In early mini heroes, if you did not water your plants they would die. In a recent update, they are no longer able to die when you leave them too long.
SEEDS: you can buy seeds in the general store for 10 paragons.
when you plant them it will take 24 hours to bare fruit. fruit can be used for crafting potions and feeding your pets.
USE: fruit is very important in the game in order to heal in dungeons without going back home to sleep, and feeding your pet so they can attack stronger.
POTIONS: as i said, potions are used for making potions. The higher level alchemy and the more fruit slots you have unlocked, the stronger your potions are. lvl 10 alchemy with 1 slot is 20 hp. lvl 10 with 2 lots is 40. lvl 20 with 2 is 80.