In short: the kernel frequency is the number of interrupts (IRQs) handled by the kernel per second. Like most applications of frequency, it's also expressed at Hz (hertz).
A more sound explanation of interrupts can be found here, but it basically boils down to this: an interrupt is generated each time an event on a piece of hardware (in the most elementary way; this includes your keyboard for instance), and requires the necessary CPU time to handle that event. This is called interrupt handling.
Now, because the kernel directly interacts with all the hardware around, it's vital to have it handle interrupts; actually, it's the kernel that handles interrupts, by using the CPU.