Public goods are valuable things (goods, information, services, etc) that cannot charge money. For example, clean air is a public good because if you pay for a factory to install pollution reducing mechanisms I get cleaner air for free. The factory cannot reduce the pollution you breathe without also reducing the pollution I breathe.
In the Ethereum ecosystem we have a lot of projects that provide public goods. For example, geth, the most popular execution layer node, underlies most projects in the ecosystem. But it has to be trusted, to be trusted it has to be open source, and because it is open source it cannot charge a usage fee.