Ethylene is an organic compound with the formula C2H4. For plants, ethylene is produced naturally by all parts of the plant including the roots, stems, leaves, flowers, and fruit. It serves as a hormone, stimulating or regulating the ripening of fruit, the opening of flowers, or shedding of leaves. The ancient Egyptians and ancient Chinese cultures discovered this and used it to help speed the ripening of figs and pears respectively.
In a food processing facility, ethylene is used to speed the ripening of plants, though it does so at the expense of the shelf life. It also shortens the shelf life of cut flowers and potted plants. This is why if you put fruit in a paper bag, you can speed up the ripening process as the natural ethylene gas that gets released, stays in a higher concentration with the bag it is in.