Harvesting is when you collect what you have grown in agriculture. Most people think of harvesting fruit, vegetables and potatoes, but wood is also harvested. "Harvesting is not only the act of collecting, but also what is collected.
One usually harvests only when the crop is ripe. Then you have to hurry: As long as the harvest is in the fields, birds and animals can eat it away. There are also smaller pests, like worms. Furthermore, it can rain: For example, if mown grass gets wet and stays out for a long time, it will mould. A bad harvest is called a crop failure.
In the past, people harvested everything by hand. That required many workers. Today, on the other hand, it is mostly done by a machine. Nevertheless, there are still fruits that are picked by hand because otherwise they could be crushed. One example is the harvesting of grapes, which is called grape picking.