/proc is a virtual filesystem (procfs) with no backing storage on disk. Every file and directory in it is generated on the fly by the kernel when read. It is the primary interface for reading kernel state and per-process information from userspace, using ordinary open, read, and write calls on what appear to be files.

Each running process gets a directory named after its PID:

/proc/1234/
    cmdline     command line as a null-delimited string
    environ     environment variables as null-delimited strings
    exe         symlink to the executable on disk
    fd/         symlinks to each open file descriptor
    maps        virtual memory map: address ranges, permissions, mappings
    status      human-readable process state: name, PID, PPID, memory usage
    stat        machine-readable version of status, used by ps and top
    cwd         symlink to the current working directory

System-wide information lives at the top level:

/proc/cpuinfo      per-core details: model, cache sizes, feature flags
/proc/meminfo      memory usage: total, free, buffers, cached, swap
/proc/uptime       seconds since boot and idle time
/proc/loadavg      1-, 5-, 15-minute load averages and runqueue depth
/proc/mounts       currently mounted filesystems (same data as /etc/mtab)
/proc/net/         network interface statistics, routing table, sockets
/proc/sys/         kernel tunables; many are writable

/proc/sys/ is the most interactive part: writing a value to a file changes a kernel parameter live. sysctl is a wrapper around this interface.

# read the current value of a tunable
cat /proc/sys/net/ipv4/ip_forward
 
# enable IP forwarding immediately (does not persist across reboot)
echo 1 > /proc/sys/net/ipv4/ip_forward
 
# the sysctl wrapper does the same thing
sysctl net.ipv4.ip_forward=1