A CustomItemPropery is, as the class name says, a custom item property that you can define.

It can then be set for your item through Item.Properties#custom.

Let's look at an example:

public class ExampleModItemProperties {

	public static final CustomItemProperty<Integer> AURA = CustomItemProperty.create(MModdingExampleMod.createId("aura"), Integer.class, 0);
}

You can then do new Item.Properties().custom(ExampleModItemProperties.AURA, 10).

And to get back the setting, just give an ItemLike instance (either an Item instance or a Block instance), to the CustomItemProperty#get method, alongside your property:

int aura = CustomItemProperty.get(ExampleModItems.EXAMPLE_ITEM, ExampleModItemProperties.AURA);