Ibid is a contraction of ibidem, a Latin word meaning “the same place.” This term is most commonly used for footnoting in scholarly texts, allowing the author to say “ibid” instead of citing a lengthy title. In legal texts, people may use “id,” a shortening of “idem,” a word which means “as mentioned previously.” If you've ever been reading a text and wondering about the identity of this “ibid” person who seems to get cited all the time, now you know!
Essentially, “ibid” is a fancy form of ditto marks. If, for example, you are referencing something like The Effects of Factory-Produced Emissions on the Greater Nile Watershed: An Environmental Study, that's a long title to have to refer to again and again. Instead, you can reference the title in a footnote, and then use “ibid” in future footnotes. If you move to a new location in the text, you can alert your readers with “Ibid (page 23)” or “Ibid, 23,” depending on what kind of citation format you are using.
When a new source is introduced, the “ibid” process begins all over again. In other words, if you cite The Effects of Factory-Produced Emissions on the Greater Nile Watershed: An Environmental Study once and follow with four additional citations marked with “ibid” before moving on to Cultural Practices in the Southern Nile Floodplain, an “ibid” after this source would refer to Cultural Practices in the Southern Nile Floodplain, not to the original text.