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.
This is a fairly fundamental concept in Java, something everyone is expected to know well about. You may want to spend some time reading about Threading. If you are a beginner just getting into Java, you will encounter this chapter eventually in your course.