The Mac currently doesn't contain an operating system, which is needed to use the device.
To get the device back up and running, we will have to install macOS on it.
This isn't hard, let's get going.
Select your preferred language.
Connect the device to power and to Wi-Fi by clicking on the Wi-Fi symbol in the top right of the menu bar.
Choose "(Re)install macOS" and click on Continue.
Click Continue.
Agree to Apple's Terms of Service.
Once more, click on Agree to accept the Terms.
Select "Macintosh HD" and click Continue.
The installation will now start. Since the operating system needs to be downloaded from the internet, this can take some time.
After the installation of macOS is finished, the device will display the setup assistant.