Complex sentence: Halliday argues that the concept of metafunction is one of a small set of principles that are necessary to explain how language works; this concept of function in language is necessary to explain the organisation of the semantic system of language.
Split into simpler sentences: Halliday argues that the concept of metafunction is one of a small set of principles that are necessary to explain how language works. In particular, this concept of function in language is necessary, he argues, to explain the organization of the semantic system of language.