What is a shutdown

Hi I’m really curious want to know what a shutdown is? I know what a meltdown is and being on the spectrum myself I’ve have had many but I have not even encountered a shutdown. My boyfriend who is also on the spectrum has them but their quiet rare. I have tried many ways to get him to snap out of the one he is currently facing but no luck instead it makes him more irritated. Basically what is a shutdown? How long do they last? And is there a way to get a person to can’t think of a less harsh phrase here but to snap out of it?