This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The lock field in the Object class is for multi-threaded programming. Using an object's lock is sometimes necessary to keep data from becoming corrupted when there are multiple threads sharing the same objects.
I consider multi-threading to be a fairly advanced aspect of Java.