When someone refers to the Ethereum ecosystem or the Cardano (ADA -4.13%) ecosystem, to use two popular examples, they're usually talking about the projects on the blockchain network. The projects can be a solid way to gauge how well a blockchain is doing and its potential as an investment.
If a blockchain has a large ecosystem with a diverse range of projects, that's a good sign. This is one reason why Ethereum is such a popular investment.
The inverse isn't always true. Blockchains that are still in the development stage may be worthwhile investments even if they don't have much of an ecosystem yet. But, at some point, it's important for a blockchain to move past the theoretical and actually attract users.