What are containers on Mac?

On Mac, containers usually refer to app containers or App Group containers in the privacy/sandboxing sense:

An app container is a protected storage area for a sandboxed app.
A group container lets multiple apps from the same developer share data securely.

Apple also mentions that data outside a container can be read by any unsandboxed process, which is why containers help protect user data on macOS.

If you meant Linux containers on Mac, Apple introduced Containerization and later Container Machine:

Containerization is a Swift framework for running Linux containers on macOS with secure isolation.
Container Machine is a new feature built on top of it that provides a persistent Linux environment on your Mac.

So the answer depends on the context:

Privacy/security: containers are protected app storage areas.
Development/server workflows: containers are Linux environments for building and running apps.