An afterimage is an artistic effect where a character appears to leave faint silhouettes behind them, which are often transparent, like a ghost. These are usually used to emphasize fast movement. A scientific explanation for this is that the character is moving too fast for the eye to track accurately, causing the brain to process images incorrectly. It could also fall under a trope feats classification.
Some fanatics tend to claim that leaving afterimages means a character is moving faster than light, despite the facts that there is no clear link between afterimages and that speed, similar effects can be observed in real world (and in other (fictional) realities) without any such speed (for example, wave your hand in front of your computer monitor), and the fact that many characters in fiction perform this "feat" when they are obviously nowhere near the speed of light (such as Marvel and DC street levelers, Rurouni Kenshin characters, etc.)
Overall, afterimages can be effective artistic effects, but they are virtually useless when it comes to determining actual speed.