Fig 1
For strong seams without puckers, the tension should be loosely balanced. The stitching looks the same on both sides because the top and bobbin threads are interlocking between layers of fabric.

Fig 2
If there are loops on the top side of the fabric:
-the top thread is too tight; loosen the top tension, or
-the bobbin thread is too loose; tighten the bobbin tension.

Fig 3
If there are loops on the botton side of the fabric:
-the top thread is too loose; tighten the top tension, or
-the bobbin thread is too tight; loosen the bobbin tension.

Fig 4
If the stitches are balance but the seam is puckered:
-the top and bobbin thread may be too tight; loosen both tensions.