TUN/TAP provides packet reception and transmission for user space
programs. It can be viewed as a simple Point-to-Point or Ethernet
device, which instead of receiving packets from a physical media,
receives them from user space program and instead of sending packets
via physical media writes them to the user space program.

When a program opens /dev/net/tun, driver creates and registers
corresponding net device tunX or tapX. After a program closed above
devices, driver will automatically delete tunXX or tapXX device and
all routes corresponding to it.

Please read Documentation/networking/tuntap.txt for more
information.

To compile this driver as a module, choose M here: the module
will be called tun.

If you don't know what to use this for, you don't need it.

source code:

drivers/net/tun.c