procfs (or the proc filesystem) is a special filesystem in UNIX-like operating systems that presents performance metrics about linux processes and other system information in a hierarchical file-like structure
The Linux monitoring tools such as top use it.
The /proc “file system” is a directory that contains a hierarchy of files that represent the current state of the Linux kernel. It allows users and applications to see the kernel's view of the system.
The /proc directory also contains information about system hardware and any currently running processes. Most files in the /proc file system are read-only, but some files (primarily those in /proc/sys) can be manipulated by users and applications to communicate configuration changes to the kernel.