The tension of the thread will affect the quality of your stitches.  You may need to adjust it when you change fabric or thread.  It is recommended that you do a test sample on a scrap of a fabric before you start to sew.

What is thread tension?

Your machine forms stitches by interlocking the bobbin thread with the needle thread.  If the needle thread tension is stronger than the bobbin thread tension, the needle thread pulls the bobbin thread through to the top.  If the bobbin thread tension is stronger, it pull the needle through to the bottom.  When the tension are evenly balanced, the stitch will lock exactly halfway between the top and bottom of the layers being sewn, which is right where you want it.

Some machines have "self-adjusting tension" so if it automatically adjusts its tension with every fabric you sew, you needn't worry about this step.

How to balance tension

Thread your machine and insert the bobbin, using two very different colors of thread, neither of which matches the fabric.  Cut a square of fabric and fold in half diagonally.  Stitch a line across the fabric. Remove the fabric and inspect the stitches.

Correct tension

Correct tension is important as too much or too little tension will weaken your seams or cause puckers on your fabric.

If the tension is evenly balanced, you will see only one color on each side.

Upper tension is too tight

Loops will appear on the surface of the fabric and you will see both thread colors on the top side of the fabric.  To solve this, reduce the tension by turning the upper tension control dial to a lower number.

Note, if the bobbin thread was incorrectly threaded, this may also cause the upper thread to be too tight.  In this case, rethread the bobbin.

Upper tension is too loose

Loops will appear on the back of the fabric and you will see both thread colors on the back side of the fabric.  To solve this increase the tension by turning the upper tension control dial to a higher number.

Note, if the upper thread was incorrectly threaded, this may also cause the upper thread to be too loose.  In this case, rethread the upper thread.