Ibid. is short for a Latin phrase that means, "in the same place," or the same as above. Chicago-style uses ibid. heavily in the notes-bibliography (NB) style or citation, and for a good reason--it makes citing in a paper so much easier!
The first time you write a footnote in Chicago style, it needs to be the long form (you can find help with that on Purdue's Online Writing Lab).
Here's an example of a long note:
1. C.S. Lewis, Mere Christianity (New York: HarperOne, 2015), 66.
The next time you use this source, you can use the short note:
3. Lewis, 89.
Now, ibid. is a special case--if your next footnote is from the *same source and page*, it would look like this:
4. Ibid.
If your note was from the same source, but a different page, this is what it would look like:
5. Ibid, 121.
So, a full example of your notes would be as follows:
1. C.S. Lewis, Mere Christianity (New York: HarperOne, 2015), 66.
2. J. R. R. Tolkien, The Silmarillion (New York: Harper Collins, 2013), 133.
3. Lewis, 89.
4. Ibid.
5. Ibid, 121.
6. Tolkien, 47.
