heat moldable ski boot shells

It appears many ‘19 boots are now “Heat Moldable” & even some carry over models now claim to be as well. Would someone educated talk me through this process?

So I'm getting ready to buy some boots and have spent lots of time trying boots on, learning about my feet, and deciding whats right for me Because I'm a poor highschool student and I'm buying my own boots, I don't have a lot of money to buy and get them fitted from a boot fitter or a good...