It means "speak" or "talk" and is used to tell someone to do so.
Here are some examples:
* ¡Hable más despacio! - Speak slower!
* Hable con su abogado. - Talk to your lawyer.
* Hable en voz alta. - Speak up.
Important note: "Hable" is used only in the formal "usted" form. If you want to tell someone to speak in the informal "tú" form, you would say "habla."