The word "render" shows up so many ways it is confusing. I'll assume you are referring to "preview rendering". It involves making previews that allow you to work smoothly while editing. The indication that you need to make or remake the preview files is a yellow bar that will show up above the timeline over sections that need it, like transitions or adjustments. If the colored line is there, the render button (or the Return key) cause PrE to make the previews. If there are not yellow lines, no preview rendering is needed.
If preview rendering is not needed, the Render button or Return cause the preview to play.
You can set whether or not your video plays after preview rendering is complete by unchecking the option in the General/Preferences, under the Edit menu on a PC.