IMDb lists all kind of titles, not just films. We also list appearances in TV specials, documentaries and other non-fiction productions. The recent surge in popularity of 'making of' featurettes, reality TV series and other productions where famous (and not-so-famous) people routinely appear as themselves had increased the size of many filmographies to the point where it was hard to tell at a glance which credits were actual acting roles.
To reduce the clutter we list some of those credits separately, based on the type of title they appeared in: performances in regular films, TV-series, TV-movies etc. are still listed in the main filmography section; appearances in documentaries, TV-specials (like the Academy Awards ceremony broadcast), making-of featurettes etc are now listed in a separate section called "Himself" or "Herself", depending on the person's gender.
Since the distinction is based on the type of title and not on the individual credit, a restricted number of appearances may show up in the wrong section. For example, all performers in a reality TV series will show up in the "Himself" category, regardless of their actual character name or role.
Most documentaries include appearances of famous people as themselves (i.e. being interviewed or in stock footage), but a few of them also feature reenactments with actors portraying historical characters. Again, these will end up in the "Himself" listing (even though the credit will still list the proper character names). You can prevent this by adding the "reenactment" keyword to such a title.