The subject line is your opportunity to get the correct experts helping you on your question. If you write "Java newbie" or "Help!!!" you won't pique anyone's interest. On the other hand, explaining your question or remark briefly on the subject line allows us to know what we are getting into, and if we will be able to help.
There are a lot of posts at JavaRanch and trying to read them all would take a lifetime, so we tend to be selective; And the subject line is where selectiveness starts. And most people don't look for posts by desperate people, but for questions that look interesting for their level of expertise.
Specifically, you don't attract experts by directly addressing them ("Foo experts, please help - it's urgent"), but by providing a well worded subject line that promises a well worded challenge ("Getting strange Foo behavior on Bar when baz").
You should also take care that you make the subject line specific enough to stand out from the rest of the posts in the forum. A subject like "Problem with Swing" or "JPanel" doesn't motivate many people to take a closer look, whereas "difference between Component and Container" (hopefully posted in the Swing forum) will attract exactly those people who can - and like to - answer.
Taking the time to compose a useful subject line shows that you care enough about your question to ShowSomeEffort when asking it here.
A subject line of "Java" is particularly useless in a forum dedicated to Java.
Many people (who would have otherwise been happy to answer you question) will skip right past something like that without ever reading it.