If you say Y here (and to "/proc file system support" in the "File
systems" section, above), you will get a file /proc/bus/usb/devices
which lists the devices currently connected to your USB bus or
busses, and for every connected device a file named
"/proc/bus/usb/xxx/yyy", where xxx is the bus number and yyy the
device number; the latter files can be used by user space programs
to talk directly to the device. These files are "virtual", meaning
they are generated on the fly and not stored on the hard drive.

You may need to mount the usbfs file system to see the files, use
mount -t usbfs none /proc/bus/usb

For the format of the various /proc/bus/usb/ files, please read
Documentation/usb/proc_usb_info.txt.

Modern Linux systems do not use this.

Usbfs entries are files and not character devices; usbfs can't
handle Access Control Lists (ACL) which are the default way to
grant access to USB devices for untrusted users of a desktop
system.

The usbfs functionality is replaced by real device-nodes managed by
udev. These nodes lived in /dev/bus/usb and are used by libusb.