It is used to explain the meaning of something, like a word, phrase, or action.
Examples:
* ¿Qué quiere decir "hola"? (What does "hello" mean?)
* Eso quiere decir que no podemos ir. (That means we can't go.)
* No entiendo lo que quiere decir. (I don't understand what it means.)
Note: "Quiere decir" is the present tense of the verb "querer" (to want) conjugated in the third person singular. In this case, it's used in a special way to convey the idea of meaning or signification.