Every change you make in the editor triggers an automatic preview render.
Preview videos are rendered at lower resolutions (up to 540p) in order to quickly show your changes in the video. If you wish to turn off automatic previews, go to Preferences in the sidebar (the wrench icon).
If you would like to preview a specific moment in the video at high quality, you can do so with an image preview. In order to issue an image preview, click 'Preview HQ Image’ button in the top right corner of the editor.