Undervolting a GPU means reducing the overall voltage that the GPU receives from the power supply. This process aims to maintain performance relatively close to stock settings while boosting efficiency by reducing power draw and heat. Undervolting a GPU wont affect performance, and you can still achieve the same clock speeds but with less heat generated, quieter fans, and overall reduced power consumption. The advantages of undervolting a GPU include:

• More efficient with reduced power consumption
• Quieter
• Less heat development
• Same clock speeds

The disadvantages of undervolting a GPU include:

• Can result in changes to performance
• Can be complicated/difficult to achieve
• Will require a lot of testing to find the sweet spot

Undervolting is typically safer than overclocking, but it does require some knowledge and tinkering to get right. Its free to do with familiar software such as MSI Afterburner, and relatively safe. Undervolting can even help prolong the life of your GPU as it will be under less thermal stress over time.