Let’s say you have a Story Template where Required field “Summary” is disabled. 

When you choose a template for creation a new Jira Issue (using left side bar menu or “X” button) - you see a warning: “Template is missing required fields”

When you’ll attempt to create an issue by clicking “Create” button - you’ll get an actual error: “summary”. You must specify a summary of the issue

The reason of the error is that “Summary” is a required field and Jira won’t let you create an issue with empty summary.

However,  if you click “Prefill” button - you’ll get native “Create Issue” Jira form prefilled with those fields that were Enabled in your template. In that case - “Description”:

Special cases:

• If you disable reporter field - Jira won’t raise an error and will set an actual person who’s creating an issue from Template as a reporter.
• If you disable “Project” field - Jira will put a project that was selected in “Easy Templates for Jira” form of new issue creation.