Good question and one when I started making homemade jam, jelly and preserves was totally clueless. I pretty much figured it out on my own through trial and error and well, if I can make it any easier for someone else then so be it!
Let's start with a wee history lesson about pectin. It was discovered by a Frenchman by the name of Henri Braconnot back in 1825. He discovered that pectin is a natural carbohydrate that is extracted from the inner peel of many fruits; it's most commonly extracted from lemons, as well as limes, oranges and grapefruits.
The peels of the fruits are washed, ground and processed to extract the pectin. The pectin is then refined, vacuum-dried and ground. Pectin however varies depending on different fruits and varieties from one season to the other as well as in progressive states of ripeness of the fruit. Just before the fruit is fully ripe is where you will find the most pectin at one time in that fruit.
When you are making your jam, jelly, preserves, fruit butters, marmalades or conserves the added commercial pectin helps to thicken them as well as to enhance level of natural pectin in the fruit. This ensures your jams and jellies "set" after shorter cooking time, not to mention gives them a more true fruit flavor than the long boiling method.
So, this is the low down on pectin. Do I see any hands raised with questions? If so let me know and I'll do my best to answer it for you.
Until next time, see you 'round the table,