Metamaterials have thrived in recent years, with negative stiffness mechanical metamaterials (NSMMS) emerging as an essential branch. Extensive research in the past decade has yielded fruitful results in this field. This work encompasses an in-depth exploration of the origin of negative stiffness behavior, along with detailed demonstrations of the implementation mechanism and construction methods used in NSMMS. Furthermore, the paper highlights the diverse range of applications for these metamaterials, including energy absorption, advanced actuators, deployable and morphing structures, vibration control, and more. Lastly, a brief glimpse into the future development direction of this metamaterial is proposed.