1. A method for controlling a mobile terminal, the method comprising: receiving content data including video data; determining whether first caption data including a first language caption is included in the content data; determining, if the first caption data is included in the content data, whether a high-difficulty word is included in the first language caption; generating explanation data corresponding to the high-difficulty word if the high-difficulty word is included in the first language caption; and converting the first caption data into second caption data by adding the explanation data to the first caption data.