A network stack is the software that lets a host communicate with the network. A host can run with an IPv4-only network stack, an IPv6-only network stack, or a dual-stacked setup.
You’re already familiar with an IPv4-only stack—it’s what most hosts ran for much of the past 30 years. An IPv4-only stack can communicate only over IPv4. Today, an IPv4-only stack gets you access to the entire Internet, with a few deliberate exceptions. That will not be true in a few years.
Likewise, an IPv6-only stack can communicate with only IPv6 hosts. Because most Internet sites don’t yet support IPv6, running an IPv6-only stack isn’t practical at this point. It is, however, an excellent way to test your IPv6 infrastructure and connectivity.