You cant go backwards, you can only go forwards.
Phil said he uses this phrase when something bad has happened or you wish you could change something. He does not know where he first heard it, but he says it reminds him of his child hood. Phil is German and grew up in a very German run household. His father was the man of the house and very strict. The basic mentality that he grew up with was you just kept going on no matter what happened, which basically captures the meaning of the proverb.
As Phil said, the meaning of the phrase is basically you have to keep going no matter what. You cannot change the past, so you two choice are to try to hide from life (which rarely works) or gut it out and move on with your life. I see this proverb as a positive idea. To me it gives hope that whats done is done and you can move on with your life. Whenever my dad would say it to me when something bad had happen, it would remind me that life does go on. What I find interesting is that Phil sees the phrase in a more negative light. When he was a child he was not allowed to hide from his problems ever. He something went wrong, his father basically told him to deal with it. For many people this may seem very harsh, but for German culture this is quite normal. The stereotype of Germans as a tough, cold people can sometimes be very true.
I also find this proverb to be very interesting because it captures a very American view point of life. Americans have been said to be future directed people. Americans focus more on whats to come, new technologies, planning out their lives, than many cultures do. This proverb captures that mentality in one statement.