What does it mean when Iran says it will begin injecting gas into centrifuges?

It means that they're taking one further step, the most significant so far, to break their own commitment to the Iranian nuclear deal. Keep in mind, the Americans pulled out well over a year ago. And with new sanctions, the Iranian economy is contracted this year by almost 10 percent. So, they're pretty angry about it, and they're lashing out. That's where we are.