What does it mean when a jar lid pops?
When being canned (or jarred as in this case) the ingredients are put into the jar hot before a lid is put on. When the ingredients cool it creates a suction that pulls the lid down. When you open the lid and hear a pop what is happening is that air is coming in to fill the low pressure inside, which indicates that there was a good seal. This is definitely a good thing. 

If there is no pop that means that there is no longer low pressure in the jar, this could be because it was never sealed properly, or the contents have gone off despite the seal and created gas inside the jar. Either way it means that it's not safe to eat.