A common feature in Canvas is the gray/green icon that indicates whether or not an item is published and thereby visible to students. Most items an instructor creates in their Canvas course (files, assignments, pages, modules, etc.) must be published before they are visible to students.
Published content in Canvas is indicated by a green check mark as published status. You can quickly review the published status of your course content and change if needed by knowing what to look for.
On the below image, the green check mark indicates the content item is published and visible to students. The grey circle with line through it means it is unpublished. To change the status just click the publish or unpublished icon:
Within the content item, unpublished content is indicated by a gray circle icon with slash through it.
You can easily switch between the two statuses by clicking on the publish or unpublish button.
See this guide on how to publish a page.
See this guide on how to publish a module.
See this guide on how to publish an assignment.