It's the second person plural imperative form of the verb "baisser", meaning "to lower" or "to decrease".
Here are some examples:
* Bassez vos voix! (Lower your voices!)
* Bassez les armes! (Lower your weapons!)
* Bassez-vous! (Lower yourself!)
If you're hearing "bassez" in a specific context, please provide more information and I can give you a more specific translation.