Eclipse has enabled spell checking by default. For some people it may be a good feature, but for many it's just useless. So if you write some word that isn't in spellcheck dictionary, Eclipse will underscore it. How to turn it off? In Java class simply point your mouse to such word and select "Disable spell checking".
You can also change this in Eclipse preferences. Go to: Window -> Preferences, choose General, Editors, Text Editors, Spelling and disable spell checking.
If you want to have spell checking enabled and you just want to change some settings, you can also do that here.