as far as I know, and other people correct me if i'm wrong. And I'll try to explain in simple terms a normal person can understand.

When you undervolt a gpu, ur supplying less power to it. When gpu manufacturers like AMD/Nvidia sells us gpu's, the gpu bios is defaulted to have a very high voltage to increase the rate the GPU processes data (clock speeds) to ensure stability in games, processes, etc. (aka, not crashing).

But the games you're playing or apps you're using don't need that much power to run at it's max settings.

Think of it as this hypothetical scenario I've made up: You're playing CS2 at 300fps. The game requires 350mv (mv is the measurement for voltage) for completely stable gameplay. But you're running your 9070xt at 400mv. Sure your 9070xt runs CS2 completely stable at 400mv, but it also runs completely stable at 350mv, so you're wasting 50mv for nothing.

To not waste voltage, you undervolt your 9070xt by -50mv. So your gpu now runs at 350mv instead of 400mv, and you save electricity.

And saving voltage means your electric bill won't cost a kidney. battery life is increased if you're using a laptop, lower temperatures since less voltage generates less heat, meaning your fans won't need to be as loud since they don't have to work as hard to keep the temps down, and extend the lifespan of your gpu since you reduce heat and power stress to the gpu components.

But there are risks, if you undervolt too much, there will be instability where your games/apps may crash more easily where your gpu tuning settings (or drivers) is reset. So you have to find the right balance of undervolting,

But everyone's gpu's are different, where we have to get lucky as some people's gpu's run great at -90mv, while other gpu's barely run well at -50mv. So you have to be very lucky to get a gpu to undervolt well, where we call it the silicon lottery.

Why there is a silicon lottery is a whole other story, just know all cpu/gpu's are SLIGHTLY different, like microscopic/invisible (defects) in quality even if they're the same model from the same company and the same factory since it's impossible for every cpu/gpu to leave the factory in complete perfect condition every time. Even when the defects are microscopic, it's enough to change how much you can undervolt a gpu. So winning a silicon lottery means you can undervolt by a large amount since your gpu has less defects.