Ibidem and idem are both Latin words, are both usually printed in italic and are both often found in reading lists, references and bibliographies. They don’t mean the same thing and they should be used in the correct context. Their respective abbreviations are similar and are therefore easy to get muddled up.
Ibidem means ‘in the same place’ but is normally used to mean ‘in the same source’ (such as a book or chapter). It is employed to avoid repeating a reference and it is often abbreviated to ibid. or ib.
Idem means ‘the same’ but is normally used to mean ‘the same person’. It is used to avoid repeating an author’s name when works by that author are cited in succession and it is sometimes abbreviated to id.
Unless you are an academic or a keen reader of non-fiction, you might not come across ibidem and idem very often. My tip if you do have to use them is to try to associate ibidem with sources that are often books and idem with a person’s identity.