I'm doing research and want to know the meaning of the word: erikoiskirjastonhoitaja. And what makes this profession?

Basically each library can decide which titles are used for the members of its staff. The title 'erikoiskirjastonhoitaja' is used mostly in public libraries. As far as I know, in Helsinki City Library the title is usually translated into English as 'specialized librarian'.

Just like librarians ('kirjastonhoitaja') and information specialists ('informaatikko'), specialized librarians are required to have a university degree in librarianship. The salary of a specialized librarian is a little higher than that of a librarian and a little lower than that of an information specialist. The middle salary of specialized librarians in Finnish public libraries in October 2013 was 2478 € per month ( http://www.kuntatyonantajat.fi/fi/kunta-tyonantajana/palkat-ammatit-ja-… ). (Cf. librarian 2328 €, information specialist 2628 €).

As the term 'specialized librarian' says, they usually have some special tasks to take care of in addition to the usual work of a librarian. A specialized librarian can e.g. take care of a music department of a library, children's and young people's library services, or any other defined sector of library services. Their tasks vary greatly depending on the library.