Common knowledge
Information a source's intended readers can be assumed to know already, and which therefore needs no citation.
Water freezes at 0 °C; the Second World War ended in 1945. Facts of that kind need no citation because no particular source is responsible for them. The boundary is audience-relative: what is common knowledge to a specialist readership is not common knowledge to a first-year one, so the same claim may need a citation in one paper and not another.
Statistics, dates that are contested, anything you had to look up, and anything an interpretation rather than a fact all fall outside it. When genuinely unsure, cite — a redundant citation costs nothing.