Zircon is the core platform that powers the Fuchsia OS. Zircon is composed of a microkernel (source in kernel/...) as well as a small set of userspace services, drivers, and libraries (source in 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.
The canonical Zircon Git repository is located at: https://fuchsia.googlesource.com/zircon
The Zircon Kernel provides syscalls to manage processes, threads, virtual memory, inter-process communication, waiting on object state changes, and locking (via futexes).
Currently there are some temporary syscalls that have been used for early bringup work, which will be going away in the future as the long term syscall API/ABI surface is finalized. The expectation is that there will be about 100 syscalls.
Zircon syscalls are generally non-blocking. The wait_one, wait_many port_wait and thread sleep being the notable exceptions.
This page is a non-comprehensive index of the zircon documentation.