you could find a description and explanation of both in the reference manual, chapter 26.5.
Synthetic keys link tables that have more than one field in common. QV creates these keys automatically.
You need at least two tables in your data model.
Though you should carefully think about avoiding these keys, you might be able to live with them.
Circular loop means you have alternative paths between two tables, so you need at least three tables in your data model to run into this. QV can't live with circular loops, it will break the loop by loosely couple one of the tables automatically.
Hope this helps,