Right now my nebulous understanding of it is that it is always going on around us, and that in some way we live in a world built on the fabric of synchronicity, from the mundane and insignificant to the extreme and mind-boggling. It is only when it rises up to the level of intellect that we give it thought or attention, but what matters is how much attention we give it. I've seen people have enormous synchronicities happen to them and have them fly right over their heads. In my own life when I'm not tuned in and a big synchronicity happens, I give it very little thought if I notice it at all.
There is a "disorder" - I forget the name but will try and get back to you with it - where people see meaning in everything that happens to them. Those would be the people who see synchronicity in everything, and interpret every leaf falling in their path as meaningful. Whether they're right or not is the mystery puzzling me right now.
As they say, 'the sea in which the mystic swims is the same in which the madman drowns,' so it's difficult territory to master.