A sewing machine works by sandwiching two layers of fabric between two threads which loop together in between the two layers of fabric. To achieve this all other cleverness has to happen, the teeth pull the fabric under the foot (which in turns hold's the fabric in place) and the teeth drop down to stop moving the fabric when the needle is down. As a tech nerd I love the ballet of mechanical cams that make all this happen in perfect time. So where does the tension come in? The tension controls how much of the top thread goes down with the needle. Different fabrics require different amounts of thread (due to how tight the weave is or how thick the fabric is). Sewing a simple test peice helps you work out how right or wrong your tension is.
In these examples the green thread is the top thread the red is the bobbin or bottom thread.
Here you can see the top thread on the top, and the bottom thread on the bottom, the threads over lapping in the middle of the fabric sandwich. A good strong neat seam.
Tension Too Low
As you can see from the underside view here the green top thread is forming loops on the under side of the fabric with the bottom thread just laying on the fabric. A terrible seam that will fall apart in moments
Tension Too High
On this top view you can see the red lower thread showing on the top. Whilst not as severe as having the tension too low this does cause problems. The fabric can become puckered around the seam and even damaged.
It's All About Balance
The bobbin thread's tension does not get adjusted, it's constant. We adjust the top thread's tension to change the balance point where the fabrics loop together. However what if the bobbin tension was not constant, behold the shoddily wound bobbin on the right.
How do we stop the dreaded thread ball?
Look at the pic on the right, look at all the technology being used to keep the upper tension even. The lower bobbin tension is set as we wind the bobbin.