term
Language models generate text by predicting what is likely to come next, so they can produce statements that read well but are not true: a wrong date, a misquoted source, a book that does not exist. This is called hallucination.
It is not lying in the human sense. A model has no built-in way of knowing which of its fluent outputs are facts.
Hallucinations become less likely when a model is given reliable source material and asked to stick to it, but the safest habit is simple: check anything important.
Example
Asked for references, a model might list a paper with a plausible title and authors that was never written.