Although you can configure WiFi using the graphical utility within the Raspbian Desktop this requires that you connect a keyboard, mouse and monitor to your Pi. It is sometimes useful to be able to do it before you’ve booted the Pi. This is especially useful when using the Pi Zero W where attaching a keyboard and mouse requires a USB hub.
The following technique will allow you to take a fresh SD card, configure your WiFi settings and boot a Pi Zero without any other wires than a power cable. It should work for both the Pi Zero and Pi Zero W but it’s easier on the W as you don’t need to worry about a WiFi dongle.