it means that on most linux distributions you get the kernel sources delivered to you in a rpm or deb package with the cds you install the distro from, but that not every distro provides them. if you need the kernel sources and you do not have the package on the cds, then you can get it from the internet...
the current running kernel can be get by opening a terminal and typing a command:
which tells the version, like 2.6.6 or whatever you are running...then just check out the installation cds for a kernel sources package with the same version, or the internet (distro's mirror sites, download sections) for the same version. the package includes linux kernel's (the core of linux) sources that can be compiled into a running kernel that has a different configuration than the one you already have, or update the kernel into a new version if you wish to do so...
