What Is Sustaining Grace?

Last Sunday, one of my pastor's preached an excellent sermon on the grace bestowed on us by Jesus, namely, His saving grace and His sustaining grace.  You can listen to it or download it here.  As I was listening to him describe Christ's sustaining grace, I was reminded of a poem of John Piper's that he expounds upon and explains during a sermon he gave on the same topic.  I think it's a succinct & helpful reminder of what God's sustaining grace is.

"Not grace to bar what is not bliss,
Nor flight from all distress, but this:
The grace that orders our trouble & pain,
And then, in the darkness, is there to sustain."