So, imagine you have a big block of ice. And you want to melt it, but you don't want to wait very long.
One way to do this is to put the ice block on a stove and turn up the heat really high. But if you do that, the bottom of the ice block will melt faster than the top, and it might even start to steam or boil.
Polybaric melting is another way to melt the ice block that helps it melt evenly from top to bottom.
To do this, you put the ice block in a container that can withstand a lot of pressure. Then, you slowly squeeze the container from the top and bottom, so that the pressure inside gets higher and higher.
When the pressure gets high enough, it makes the ice block start to melt, even though it's still pretty cold. This happens because the pressure is making it easier for the ice to turn into water.
As the pressure keeps increasing, the water from the melting ice starts to mix with the ice that hasn't melted yet, making it easier for that ice to melt too.
Eventually, the whole ice block will turn into water, and it will have melted evenly from top to bottom.
It's kind of like squeezing a toothpaste tube from the bottom – the toothpaste comes out more easily when you do that, because the pressure helps it flow. Except with polybaric melting, you're squeezing ice instead of toothpaste!