Sharecropping is of course when a landowner lets someone else farm their land in exchange for a share of the crops they produce. It has the usual tradeoffs, incentives for each party, and dysfunctions at the extremes (like demanding 99.9% share) that any landlord/renter system has. I think I originally came across the expansion to a digital concept here https://blog.codinghorror.com/are-you-a-digital-sharecropper... (which is really just aggregating a couple other links), it was more focused on the phenomenon of social media users creating most of the value (usually without any monetary compensation) for the social media site owners (who did sometimes succeed in converting that to money, though as we see even in Current Year it's not exactly trivial to turn a profit).

Extending the idea of sharecropping again to digital services is just a way of describing a service or business that existentially depends on some other service (or platform or API) not just for survival (so mere dependence on e.g. legitimate payment processors doesn't make one a cropper) but also for the source of most of the value it hopes to expand upon and capture a part of. If that dependency is broken it wouldn't make sense or be feasible to have the service standalone. Social media integration startups are quintessential examples, the corpses are many. What's the point of some random startup that crawls twitter data to e.g. present rich marketing analytics data to its customers if twitter shuts down, or loses almost all its users except for like 1000 randos, or cripples its APIs among other things to make such mass data ingestion infeasible, or directly goes after the startup with lawsuits, or acquires a competitor and integrates equivalent-or-better analytics natively?

Sharecropping isn't necessarily a bad arrangement for either party (and there are cases it can be bad to be the owner). Some companies (e.g. Salesforce with its AppExchange marketplace) thread the needle a lot better when it comes to having a useful platform/pool of existing value for third parties to make useful extensions to and not scaring off new croppers or killing off many existing ones. And some croppers are more careful in selecting whose land to crop on or making their cropper status less of an immediate danger by being able to crop on multiple owners' land at once.

It means that you're farming on someone else's land. Since you don't own the source of your income, the landowner could decide to evict you at any time, taking away your revenue.