The recently proposed orthogonal time frequency space (OTFS) modulation has demonstrated superior performances over the conventional orthogonal frequency-division multiplexing (OFDM) modulation. In this paper, we provide a tutorial to the OTFS modulation for future wireless communications. We first revisit the properties of the wireless channels and then introduce the OTFS system models. In particular, we also discuss the characteristics of OTFS signals over different domains. Based on the OTFS characteristics, we briefly discuss the performances of OTFS modulation, including the performances of diversity and coding gain, channel estimation, and detection. Finally, we provide a discussion regarding the potential applications of OTFS modulation for wireless communications, where the research challenges are also highlighted.