Heat-moldable means that you can use convective heat (heating by hot air, like a blow dryer. They have special boot warmers at shops for this) to warm the material in the boot liner and it become malleable, or you can change the shape to fit your foot more snugly.
You can use a hair dryer or put some in the oven at very low temperatures for about 10 minutes. Then put the liners back in the boot, buckle them down tight and they shape to your foot as they cool. Make sense?