Ignition is a new provisioning utility designed specifically for CoreOS Container Linux. At the the most basic level, it is a tool for manipulating disks during early boot. This includes partitioning disks, formatting partitions, writing files (regular files, systemd units, networkd units, etc.), and configuring users. On first boot, Ignition reads its configuration from a source-of-truth (remote URL, network metadata service, hypervisor bridge, etc.) and applies the configuration.
Even though Ignition only runs once, it packs a powerful punch. Because Ignition runs so early in the boot process (in the initramfs, to be exact), it is able to accomplish configuration all before the userspace has begun booting, which enables advanced features that Container Linux admins require.