The word "that" is a conjunction used to introduce a subordinate clause, which provides additional information about a noun or verb in the main clause. For example, in the sentence "She said that she would come," the clause "that she would come" gives more detail about what she said. In addition to its use as a conjunction, "that" can also function as a pronoun or a determiner. As a pronoun, it can refer to a specific thing or idea, as in "I like that." As a determiner, it can specify a particular item, such as in "that book."