Negative symptoms in schizophrenia are reductions in usual experience and behaviour, such as less facial expression, less speech, low motivation and reduced pleasure or social contact.
'Negative' here means something is reduced or absent. It does not mean bad. Positive symptoms are experiences added to usual experience, such as hallucinations or delusions.
Negative symptoms matter for autism assessment because some of them can look similar from the outside. Reduced facial expression, a flat tone of voice and preferring time alone are also common in autistic adults. The difference is history. Autistic traits have been present since early childhood. Negative symptoms usually develop later, often in late adolescence or early adulthood, as a change from how the person used to be.
If you have a psychosis-related diagnosis and wonder whether autism was missed, raise it with your treating team.