Drop chance is a determining factor in Growtopia. The drop chance [of a tree or item] is calculated based on how much fruit a single tree can drop. This is usually in multiples of four because a normal tree can display four fruit in its tree, but certain items (like the Legendary Wizard) have a drop chance of one.
It is used to determine if an item can be farmed or not. The higher the drop chance, the easier it is to farm as more of the item will be given every time it is farmed.