Harvesting
Harvesting is the act of picking up items, generally (and in this case) from plants or trees.
How to Harvest
Harvesting Plants
The first way (to harvest from plants) is to use a machete or spade to destroy the top half of the plant once the plant has grown to be two blocks tall. This is the most productive way to harvest seeds or foods from plants, and if you use a tin spade your tool won't even take any damage.
The second way to harvest (from plants) plants is to wait until the plant is full grown for a while, and then the top portion of the plant will automatically "die" and fall off. When this happens, one seed or food item will fall, and you can collect it. This is an easier way, but it takes quite a bit longer to collect more foods.
Harvesting Trees
Harvesting from trees is done a little bit differently. Trees grow like trees, and so they start of as small bushes. After a while, they'll grow and then grow some more, and eventually they will reach "maturity" and will start producing foods. When trees produce foods, the items - the food for the type of tree you planted - will spawn hanging off of the branches of the tree every once in a while. You should have your blockhead pick these up, since it is the only food the tree will produce, and after a while the foods de-spawn (vanish). As the tree continues to grow, it will get more branches and grow taller. The branches can be destroyed using a machete to give your blockhead 3 sticks. The middle of the tree, where the branches don't have air showing between them but instead show wood (the tree trunk), is where you can destroy using an axe to collect wood. It is best to keep your trees small (but not too small) since they will both grow faster, and live longer as long as you do so.