It means working from 9 am to 9 pm, 6 days a week.
It started to gain popularity when some Chinese company, predominately tech companies (and game companies) would openly require their workers to work for 12 hours a day, 6 days a week, without overtime pay.
It is listed on their hiring ads.
It is, of course, against Chinese labor law. But speaking from the gaming industry (since I’m more familiar with it), nobody enforces the law. And the companies would say the workers volunteered to stay late to finish the work.
What came after 996? ICU.
you work that much with that kind of intensity, you end up in ICU sooner or later.