“The file /bsd is OpenBSD’s kernel. Next question?”
That’s technically correct, but not exactly useful. A more general description is that “The kernel is the interface that links applications and the hardware.” That’s not a complete definition, but it’s good enough.
The kernel allows programs to write data to disk drives and to the network, and it gives instructions to the CPU and shuffles bits into memory. When you open a web page, the browser application asks the kernel to fetch the data it displays.
Some kernel responsibilities exceed this definition. For example, the kernel handles network connectivity, including forwarding packets from one interface to another if needed. The packet-filtering rules run in the kernel (although ...