Startups go through different growth periods and a later-stage company is situated at the other end of the spectrum, opposed to an early-stage one.
The later-stage companies proved their concept, reached a level where their product is well-known, have established viability, achieved strong revenues and a strong base of constant customers, and have probably reached a positive cash flow. They might have even reached profitability or are near that point.
When it comes to investments, this positions them in Series C, Series D, or later rounds. The company is at a point in its life where it might consider acquisitions, an initial public offering (IPO) or other exits. While investing in startups is risky regardless of the stage of the company, it seems that investing in a later-stage one is among the safest types of investments, offering the most assurance.
Any company has its unique path, so the details above are not universally valid. At any point in time, even for the later-stage companies, unexpected events can occur and derail it from its growth paths – such as the COVID-19 pandemic, economic slumps, unusual market trends, and many more.