Dell (and a select few of other display vendors) have something they call a "fast" response mode. Which is merely a software setting that essentially "overclocks" your monitors refresh rate (see @MysteriousAeon's reply for an explanation of what refresh rate is.) This allows for a faster updating of the pixels whilst significantly worsening the quality of the image. Ghosting and other artifacts are not uncommon when "overclocking" your display.
I'd recommend that you don't use this feature and keep the display at its normal settings to avoid such artifacts.