With both the kernel and modules built and installed, you are ready to let the system use the new kernel.
However, until you reboot you are still using the old kernel, so now is the time to make sure you have done everything correctly.
If you implemented certain kernel components as modules, specifically those required at boot time, you will need to provide access to them through a ramdisk
The ramdisk holds an image of the modules, and the kernel loads that image into memory before any other components.
However, ramdisks store their data in random access memory, as opposed to on disk. This is useful during installation.
When the system reboots, the kernel notices you have a ramdisk for the new kernels version.
It then loads the images in that file and continues to boot.
There are no restrictions on what to name the initial ramdisk image, but meticulous naming helps in the long run.