The abbreviation "ibid." stands for the Latin word ibidem, which means "in the same place."
You may use "ibid." in a footnote when the source you're citing is the same as the footnote right before it, and thus avoid writing out the citation information again. If the page number is the same, just the abbreviation is enough. If the page number has changed for this citation, you should include the new page number after "ibid."
Let's say you are citing the same book four times in a row in your paper. Here is how your footnotes should look:
1 Natalie K. Watson, Feminist Theology (Grand Rapids, MI: W. B. Eerdmans Publishing Co., 2003), 47.
2 Watson, Feminist Theology, 62.
3 Ibid.
4 Ibid., 63-64.
The first citation is the first time you have cited the book in your paper. The second citation is the second time you have cited the book. The third citation shows that you are citing the same book and the same page number. The fourth citation shows that you are citing the same book, but different page numbers.
Note that citations #1 and #2 could have citations of other sources in between them and still look exactly the same, but citations #3 and #4 must follow citation #2, because otherwise, your reader will not know what source you mean when they see "ibid."
If your paper is organized by section headings, you should not use "ibid." as the first citation in a new section, even if the last thing you cited was from the same source. Instead, use the short-form citation in the footnote #2 example.