Publishing makes a template available to end-users as a design generator: the form where they fill in your input fields and produce their own version.
Publish a single template
Open the template in the editor and click Publish. The template goes live as a design generator and you can copy a (customizable) link to share it.
Publish through a Campaign
You can also publish templates as part of a Campaign. Campaigns let you bring several templates together and offer extra configuration beyond a standalone published template. It can even be worth creating a Campaign for a single template when you want those options. See the Campaigns section for the available types and how to set them up.