When the HD image options is switched on, it will double the resolution of the original template.
For example, if a template has a resolution of 1200 x 624 pixels, the image processor will render an image of 2400 x 1248 pixels.
This option will have your featured image looking crisp on high DPI displays and it is recommanded to activate it since WordPress natively handles well big images and will render a scaled down image depending on the device that is being used.
When using images from Unsplash, our engine will automatically use higher definition images to accomodate the HD rendering.
However, if you use your own images, make sure their resolution is big enough (ideally at least double the size of its container) or the result may end up slightly blurry.