Parenting is how you get an object into an Empty. The Empty in Mecabricks is kind of like a group, except that it has a defined point in space. This is useful for animations. In the animation node editor, you need to be able to select objects (which includes bricks and Empties), but you can't select groups. So if you put your parts into an Empty first, then you can animate the parts together by animating the Empty.

This link has a discussion of Empties and Parenting as it relates to Blender, but it works mostly the same in Mecabricks. (Only need to read thru the Parenting section).
[url]https://blender.stackexchange.com/questions/45848/what-is-an-empty[/url]