What is a "stake?"
A stake is often used to describe the amount of stock an investor owns, and this is certainly a correct way to use the word. If you own stock in a given company, your stake represents the percentage of its stock that you own.
However, a stake doesn't necessarily need to refer to stock ownership. Rather, "stake" is a more general term used to convey partial ownership in a company. As an example, if you and a business partner decide to buy an investment property together, you could say that you both own a stake in the property even though there's no formal stock structure. In addition, bondholders are considered stakeholders in a company because they stand to benefit if the company performs well.
Additionally, if you invest in a smaller, non-public company, you might receive a stake in the business in exchange for your investment. Let's say a company is looking to raise $50,000 in exchange for a 20% stake in its business. Investing $50,000 in that company could entitle you to 20% of that business's profits going forward.