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

It can then be set for your block through BlockBehavior.Properties#custom.

Let's look at an example:

public class ExampleModBlockProperties {

	public static final CustomBlockProperty<String> SHOUT_MESSAGE = CustomBlockProperty.create(MModdingExampleMod.createId("shout_message"), String.class, "Archeon is delayed...");
}

You can then do BlockBehavior.Properties.ofFullCopy(Blocks.STONE).custom(ExampleModBlockProperties.SHOUTH_MESSAGE, "I want cupcakes!").

And to get back the setting, just give a Block instance to the CustomBlockProperty#get method, alongside your property:

String shouting = CustomBlockProperty.get(ExampleModBlocks.EXAMPLE_BLOCK, ExampleModBlockProperties.SHOUT_MESSAGE);