You get a Stock Bonus whenever you fill the Stocks bar on a Business. Those bonuses award payout multipliers for that Building, as well as a small amount of Energy Drinks and/or Manager's Cards.

        The orange meter indicates your progress towards the next Stock Bonus, after buying more stocks. The buy button displays how many stocks you can buy at once, and how much it would cost.

        If a Business meter is completely full, it will appear green, which means you've already earned all its bonuses. Keep in mind that you can still buy more stocks to increase the Building payout or to complete certain goals.