Negative Levels are a concept appearing only in the Yu-Gi-Oh! 5D's anime, and are used on the anime and video game exclusive Dark Synchro Monsters. In real life any such Levels are instead positive.
The only card that can cause a monster to have a negative Level (in the anime) is "Dark Wave".
They are represented by , which aligns to the left of the card, whereas positive Level Stars align to the right. This symbol is the color inverse of a positive level.