"Olam's strategy has been based around transitioning from an asset-light supply-chain trading company to an integrated global agri-business. Upstream investments in plantations, as well as downstream ones in manufacturing, brought greater synergy and helped Olam to differentiate itself from competitors such as global trading giant Cargilland regional commodities player Wilmar, Verghese says."
Upstream is into businesses that supply the original business; downstream is into businesses that make use of the original product. So in that description, what they are saying is that the original business received products from plantations and sent products to manufacturing.
This is also called vertical integration. Meaning that they are diversifying along their supply chain so that they control more of it. This is in contrast with horizontal integration, where they move into new products that either compete with the existing products or which are entirely separate.
In general, the upside of vertical integration is that a company is less reliant on suppliers (and intermediate consumers) and has more control over its supply chain. The downside is that they have less opportunity to partner with other companies in the same supply chain, as they compete with them. Some companies are better at managing to do both. For example, Amazon.com has integrated fulfillment and sales. But partners can still do their own fulfillment and/or sales, choosing how much to send out to Amazon.
If you are investing in individual stocks, integrated companies can be problematic in that they cut across diversification areas. So they can be harder to balance with other stocks. You can either buy plantations, transport, and manufacturing together or not buy at all. If your investment strategy says to increase plantations and reduce manufacturing, this can be difficult to implement with an integrated company. Of course, everyone else has the same problem, which can lead to integrated companies being undervalued. So they may be an opportunity as a value stock.