The ZIL is the "ZFS Intent Log", a log-structured ordered stream of file operations to be performed on the ZFS volume.
If power goes out, or the disk controller goes away unexpectedly, this ZIL is the log that will get replayed to bring the volume back to a consistent state. I think.
Usually the ZIL is on the same storage devices as the rest of the data. So a write to the ZIL has to wait for disk in the same line as everybody else. It might improve performance to give the ZIL its own, dedicated storage devices. NVMe is great, lower latency the better.
Since the ZFS Intent Log gets flushed to disk every five seconds or so, a dedicated ZIL device doesn't have to be very big. But it has to be reliable and durable.
Windows made small, durable NVMe cache drives a mainstream item for a while, when most laptops still used rotating hard drives. Optane NVMe at 16GB is cheap, like twenty bucks, buy three of them and use a mirrored pair of two for your ZIL.
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Then there's the read cache, the ARC. I use 1TB mirrored NVMe devices.
Finally, there's a "special" device that can for example be designated for use for intense things like the dedupe table (which Fast Dedupe is making smaller!).