Previewing a document allows you to view what that document's audience will see after you publish or launch your current draft document. You can do this from a few different locations:
Your Home Page or the Document Library
Click the gray down arrow button next to the gray "Edit" button (or next to the gray "Write, Edit, and Design" button for an unlaunched document), then choose Preview.
Within the Document Editor
Click the gray "Preview" button in the top right corner.
From the Launch Checklist
Click the blurple-outlined Preview button under the CREATE task.
Options
After you open the preview, check out the dropdown at the top of the page in the preview bar. It allows you to choose the audience member to preview as. This is particularly helpful if you have personalized content and you want to see what different audience members (employees) will see, so you can verify that people are seeing what they're supposed to see.
If you need to share this preview with someone who's not an editor, you can do that too (and even share the different personalized versions). Check out our help article on how to share a document with a stakeholder before you publish.