While fixing some issues in Kernel or while debugging Kernel or modules, it is possible to introduce errors into the Kernel source. Errors in the Kernel may lead to failure in booting. To avoid such failures, FreeBSD has a feature in the Kernel installation/loading which allows you to testload a Kernel i.e., just to load that Kernel once. Second boot will use the stable Kernel. You can find more details in the src/UPDATING file.

make installkernel KERNCONF=<updated_kernel> KODIR=/boot/testkernel nextboot -k testkernel