The Pi Zero (and Zero W) have support for acting as a USB gadget: that means that they can be configured to act as a USB device – like a serial port, an ethernet interface, a mass storage device, etc.
There are two different ways of configuring this support. The first only allows you to configure a single type of gadget at a time, and boils down to:
- Enable the dwc2 overlay in
/boot/config.txt - Reboot.
modprobe g_serial
This process is more fully documented here.