This is a virtual file system providing information about the status
of the system. "Virtual" means that it doesn't take up any space on
your hard disk: the files are created on the fly by the kernel when
you try to access them. Also, you cannot read the files with older
version of the program less: you need to use more or cat.

It's totally cool; for example, "cat /proc/interrupts" gives
information about what the different IRQs are used for at the moment
(there is a small number of Interrupt ReQuest lines in your computer
that are used by the attached devices to gain the CPU's attention --
often a source of trouble if two devices are mistakenly configured
to use the same IRQ). The program procinfo to display some
information about your system gathered from the /proc file system.

Before you can use the /proc file system, it has to be mounted,
meaning it has to be given a location in the directory hierarchy.
That location should be /proc. A command such as "mount -t proc proc
/proc" or the equivalent line in /etc/fstab does the job.

The /proc file system is explained in the file
Documentation/filesystems/proc.txt and on the proc(5) manpage
("man 5 proc").

This option will enlarge your kernel by about 67 KB. Several
programs depend on this, so everyone should say Y here.

source code:

drivers/pci/proc.c