Content knowledge comprises the amount and organisation of subject-related knowledge in the teacher's mind (Shulman 1986:9; 1987:9). Roberts (1998: 105) asserts that it means that teachers show knowledge of the systems of the target language and their communicative competence in it. In other words, teachers are expected to have declarative knowledge of the language (Bailey et al. 2001: 23), i.e. knowledge about English grammar and phonetics, for instance, and be proficient and confident users of it as they are language models for their learners to follow (Barnes 2002:199).
You can read more about content knowledge at http://www.elted.net/5%20Dario%20Luis%20Banegas.pdf