The kernel is the core of the operating system that provides the basic services that programs need such as multi-tasking (process management), memory and virtual memory management, access to the filesystem, device drivers (or access to them), networking and interprocess communication.
The FreeBSD kernel is primarily a monolithic kernel meaning that most of its core services are tightly integrated and run in kernel space. It is also flexible in that it can be extended while running, with external modules using the kldload command. In a monolithic kernel, the services the kernel provides are made available through a set of system calls to programs that need to access them.
You may at some point wish to build your own custom kernel.