The little text is Furigana or in wider term, like StrangeStripes said, a Ruby.
If you see most of chuuniby-- *cough* fantasy japanese story, either manga, novel or light novel, this will be used to put a "cool" term, that the author wants to use, and the (kinda) simple description on it. Here's one general example from Fate series :
The kanji read as : yakusokusareta shouri no ken, but the term that will be use by the author/mangaka is the small text : excalibur
Weird? Like certain anime priest once said : "you get used to it".