Patches do not normally pass directly into the mainline kernel; instead, they pass through one of one-hundred or so subsystem trees. Each subsystem tree is dedicated to a specific part of the kernel (examples might be SCSI drivers, x86 architecture code, or networking) and is under the control of a specific maintainer. When a subsystem maintainer accepts a patch into a subsystem tree, he or she will attach a “Signed-off-by” line to it. This line is a statement that the patch can be legally incorporated into the kernel; the sequence of signoff lines can be used to establish the path by which each change got into the kernel.
See: http://www.linuxfoundation.org/publications/whowriteslinux.pdf