When you see ibid. in a list of references or citations, it's a shortcut that lets you know the given information was found in a previously mentioned source.
The word ibid. is actually short for the Latin word ibidem, meaning "in the same place." It is used in citations to refer back to the same source previously cited. This is handy in scholarly work when the same book or article is cited multiple times in a row. For example, if you're writing a paper and use several different facts from the same book, using ibid. can save you from repeating the full citation each time.