A berry is a fruit that usually encloses many seeds of the plant. When berries are ripe, they fall off the plant. They are then still closed and juicy and are therefore also called a closed fruit by biologists. If the fruit falls on good soil and remains there, the seeds sprout. Then new plant can start to grow.
Berries, however, have another function in reproduction: animals or humans eat the fruits. They usually cannot digest the seeds. So they are excreted again with the faeces and then grow in a faraway place, the faeces being there as fertiliser at the same time. This way the plant can spread much better.
Usually we mean by berries only small, soft, sweet fruits, i.e. fruit varieties such as raspberries, blackberries, strawberries or currants. Berry fruit is often also brightly coloured. Herbalists are stricter with the term: for them, raspberries, blackberries and strawberries are not berries. But plant scientists also count bananas, oranges, kiwis or even melons as berries. Even vegetables such as tomatoes, peppers, pumpkins and cucumbers are berries.