Zircon is the core platform that powers the Fuchsia OS. Zircon is composed of a kernel as well as a small set of userspace services, drivers, and libraries in /zircon/system necessary for the system to boot, talk to hardware, load userspace processes and run them, etc. Fuchsia builds a much larger OS on top of this foundation.
See the documentation at: https://fuchsia.dev/fuchsia-src/zircon