What does a lockdown actually do?

[Former user]

The FAQ doesn't clearly explain what a lockdown actually does. Should be reworded.

What does it mean when a lockdown occurs? Lockdowns randomly occur to every user. They are limited to every 12 hours, so if you get one at 12:00 PM (Eastern) you won't get another until at least 12:00 AM, but usually they'll happen farther apart than that since they are random.

That just says they happen randomly and once every 12 hours minimum. But what happens? The question asks "what does it mean" and doesn't answer that question.