This statistic shows kernel memory which is currently lost to fragmentation, and can be broken down by kernel cache (kmem cache). Such a state can occur when memory is freed (for example, when cached file system data is deleted), and the kernel has yet to recover the memory buffers.
An understanding of operating system internals is required to understand this statistic.
Rarely. If the dashboard were to show kernel memory as a large consumer of available DRAM (in the Usage: Memory section), then this may be used when troubleshooting the cause. Also see Memory: Kernel memory in use.