What is MQ and how is it used in an OpenStack deployment and Java EE applications?

I have a job interview in a couple days and they are looking for experience with MQ in an OpenStack and Java EE environments.

I have lots of Java and Java EE experience, and I've done some basic OpenStack tutorials, but I don't really understand what "MQ" is and what it's used for.

Can someone give me a good answer: What is MQ and how is it used in an OpenStack deployment and Java EE applications?