A saving grace is something positive in an otherwise bad situation. You might say that the saving grace of your long, uncomfortable car trip to your grandparents' house is being able to watch movies on your laptop.
The saving grace of a boring class could be the fact that your best friend sits next to you and sneaks you funny notes, and the saving grace of a badly cooked meal might be ice cream for dessert. The saving grace is the good thing that redeems, or saves, a negative experience. The original meaning, and one still used today, is religious: God's grace or blessing, especially when it's granted suddenly and "saves" a sinner.