This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
That just means that you have not attached the source to the JARs in your build path. As to whether it is a problem, it is only if you want to see the source If you do, attach the sources. To do this you have to first get the sources and then expand your project to see the included JARs. Right click on a JAR to get the info about it and one of the options will be to attach the source.