Gathering means collecting plants that grow wild for food. In the Middle Stone Age, until around 8000 BC this was the main source of food. People gathered grain (wheat and barley and millet and rye), fruit like apples and pears and figs, olives, wild carrots and peas, lentils, mushrooms, berries, chestnuts, walnuts, almonds, herbs like oregano, thyme, and mint, and berries: strawberries, raspberries, blackberries, currants.
Around 10,000 BC, in the New Stone Age, people began farming, and therefore spent less time gathering. Now they grew grain instead of gathering it. People soon also began to grow parsnips and peas and lentils, but they continued to pick mushrooms, herbs,
berries, apples, pears, olives, and nuts. By the Bronze Age, around 3000 BC, people planted more kinds of food and gathered less. Bronze Age people planted grain and vegetables, but they also planted fruit trees and olive trees. Still, they picked nuts, mushrooms, herbs, and berries in the wild.
In fact, the situation has not changed much from then until now. Even today, many people still gather their berries and nuts, and some people still gather mushrooms and herbs.
It might seem like gathering is a rather lazy, disorganized sort of way to get food, and doesn't need much knowledge. But really it is very complex. You have to know where the plants are, like where are the nut trees growing and where are the berry bushes. You have to keep track of when each of them will come ripe, so you don't accidentally miss all the berries by not visiting their bushes the right week. You have to arrange things so that you are in the right place at the right time.
Then also, once you have picked the food you have to preserve it and store it: you have to thresh the grain, and dry the berries and fruit and herbs, and pickle the olives. Usually people who get most of their food from gathering have a regular route that they travel around every year, designed so that they are at the nut trees when the nuts get ripe, and at the olives when the olives get ripe.
But even with all this, studies of gatherers suggest that most gatherers only really work at gathering food for a few hours a day, much less than farmers work. That is, people began farming not because it was easier, but because it could feed more people on smaller patches of land, when climate change made some land into deserts where nobody could live.
For more information about gathering, check out these books from Amazon.com or from your library:
The Cambridge Encyclopedia of Hunters and Gatherers, edited by Richard Lee and Richard Daly (1999).By specialists, but accessible to interested adult readers.
Woman the Gatherer, by Frances Dahlberg (1983). A collection of essays.
Neanderthals, Bandits and Farmers: How Agriculture Really Began, by Colin Tudge (1999). An attempt to explain why people gave up gathering and began farming. Short, and not too hard to read.
Last Hunters-First Farmers: New Perspectives on the Prehistoric Transition to Agriculture, by T. Douglas Price (1995). A more academic book on the same problem.