Now, attempt to drag the box part. This part does not move, and your cursor changes to the push-pin, grounded symbol.
Autodesk Inventor always grounds the first part placed into an assembly to provide a fixed, foundational part to which you can constrain other parts. You can specify that any part is grounded or not grounded. You can also specify that the first part placed is not grounded. But the best practice is to select a strategic, foundational part for the first part placed, and then leave it grounded.