When learning to use your overlocker/serger, it's important to understand which thread is which and what part it plays within the seam. This information will help when you need to adjust any tension issues.
A great way to learn what thread does what, is to load a different brightly coloured thread into each of the spool stands and sew a test piece on lightly coloured fabric. Refer to each coloured thread and identify it's name - in my example, red is the upper looper thread. (my tension may appear a little off and that's because it's set to sew with pvc and that play's havoc with tensions!)
Your machines manual will have detailed diagrams plus most will have a diagram placed somewhere on the machine showing each of the threads name (mines inside the looper cover as pictured but each machine will be different).