Any text document, source code included, is composed by a set of characters. Not all those characters are visible. For example, a white space is normally invisible, as the same as the tab (normally \t) or End of Line (EoL).
As in Word, the eclipse can show those invisible characters. To turn on them
1. Open preferences in eclipse
2. Go to General -> Editors -> Text Editors
3. Enable the function "Show whitespace characters"
You might notice that "whitespace characters" has been underlined and it is blue. This is because it is a link. If you click on it, you can see all the options that it has.