What is a passive loudspeaker radiator?

Some models of loudspeakers feature multiple woofers where some are active while others are passive and don't make a sound on their own. What are these and how do they work? Have a question you want to ask Paul? http://www.psaudio.com/ask-paul/ I am getting close to publishing my memoir! It's called 99% True and it is chock full of adventures, debauchery, struggles, heartwarming stories, triumphs and failures, great belly laughs, and a peek inside the high-end audio industry you've never known before. I plan a few surprises for early adopters, so go to https://www.paulmcgowan.com and add your name to the list of interested readers. There's an entire gallery of never before seen photos too.