It's very subjective. You can read more about it by searching around, but the prevailing definition means people who had more severe reactions to the infection than the average patient, which almost unanimously means a severe and prolonged inflammatory response.

What they are finding more and more is that this specific virus triggers undiagnosed or dormant autoimmune responses in patients. If you're familiar with Rheumatism at all, imagine a massive reaction to, say, twisting your ankle, but it goes out of control and causes swelling all over your body including your brain, lungs, heart, and renal system. This is actually what killed the most patients pre-vaccination.

The full body assault of an inflammatory response just makes the body unable to cope, and things start shutting down. It can kill children and older people very easily if you can't get it under control. Many succumbed to Pneumonia, but those that lived had damage to their major internal organs from the inflammatory response. You can also see some had been using the term "Walking COVID", meaning people who had cleared the infection, but had long term sustained symptoms similar to Emphysema. They've since just moved on to calling everything "Long COVID". Some people recover, some people don't.

This study finally identified the specific damage and detection to brain activity.