Embed templates are presets for creating embeds. With them you can save the settings of an embed and re-use them on every new embed you create.
To create a new template open the embed wizard and create a embed of your liking. Then, after you configured the embed, click on "Save as template" on the bottom of the embed wizard. Give the template a name and hit "Save".
The template can now be chosen in the drop down list in the top of the embed wizard.
Learn how to delete an existing embed template here