A public good is product, service, or information that is non-excludable (people or groups cannot be effectively prevented from consuming it) and non-rivalrous (one person's consumption doesn't prevent simultaneous consumption by another). Public goods are most often associated with governments and nature.
The opposite is a private good. It's an item used for private benefit that is owned by someone or a group (e.g. they can exclude others with enforceable property rights). When someone consumes a private good, it prevents anyone else from doing the same (rivalrous). Examples include food, clothing, and private land.
Many products and services exist on a spectrum between a pure public good and a private good. Highway systems are often cited as a public good, but anyone who's experienced big city traffic knows that there is a limit to how many people can simultaneously use a road. It's a matter of physics.
Informational goods aren't constrained the same way. Information like scientific knowledge, mathematical equations, and software code can be replicated with effectively zero marginal cost. They need not be rivalrous or excludable, but can be made so by choice.
The fixed thinking of private goods betrays the opportunity offered by the Information Age. On the internet, we have the ability for more and more goods and services to be public ones. Blockchain technology, cryptography, and mechanism design are exciting building blocks for creating better digital public goods. If we do so, we can create a future of abundance and mutual benefit for all.