Lecture #11 looked at some smarter undirected search mechanisms and heuristic search.
Notes are available in two formats:
The lecture covered the material at the end of Chapter 8 and various things from Chapter 9, broadly speaking pages 135-136, 139-145 and some of the material from 155-160.
You should go through this material in conjunction with the notes (but be aware that the last part of Chapter 9 will not make complete sense until you have covered the material in 145-155).
The next lecture will (amongst other things) go through the material in the remainder of Chapter 9, so reading through pages 145-155 would be a good idea. You don't need to understand every step of the proofs (though if you can, great) but try to get why the Theorems are important and what the intuition behind them is.