Can someone ELI5 on how Transfigure works on the Horadric Cube?

Hi all, not smart person here trying to figure out how some sections of the Cube works. I understand other parts of the cube but I see alot of posts here about how people are rolling +2, +3, or +4 GA'd items from previously blue or yellow ancestral items they've found. Could someone give me a simple explanation of how the cube works to create items like this? I feel like I'm missing out on crafting good gear I otherwise would have kept by using this feature. Thanks!