Tension! the biggest fear of many machine owners sewing life. Tension is the cumulative pull on the thread as it travels to form the stitch.
There should be very little reason to adjust tensions on your machine if it is in good working order, has been threaded with the presser foot up (so the tension disk is open to seat the thread correctly) and has the correct stitch setting, threads and needle for the fabric you are using.
Your machine is probably configured to sew with a 50wt cotton or 60wt polyester thread and a size 80/12 universal needle and sewn on a medium weight cotton or calico. replicating this is the easiest way to check your tension.
Use contrasting colours top and bottom and see how the machine sews in a straight stitch and a zig zag stitch.
Bottom colour showing on the top is too high a tension on top so lower the number to 3-4.
Top colour showing on the underneath so raise the number to 5-6.
If you are constructing a garment, cushion, curtains or a quilt you need to be using the same thread top and bottom to have a quality stitch that locks you seams properly.
When you want to do quilting or decorative stitches and use different weight of threads in the bobbin you may need to either loosen the top tension OR tighten the bobbin tension*.
Many machines have special 'high tension' bobbin cases or the ability to increase the bobbin thread tension by a set amount.
* We do not recommend 'fiddling' with bobbin case tension unless you are using a spare. (Your service engineer may have taken a lot of time to balance your bobbin tension perfectly.)