A safe-haven currency is one that investors tend to buy specifically during periods of uncertainty or market stress, largely independent of that country's own economic data on the day. The US dollar, Japanese yen, and Swiss franc are the three most consistently cited examples — for different structural reasons in each case (the dollar's role as the world's reserve currency and most liquid asset; the yen's role as a funding currency that gets bought back when carry trades unwind; the franc's association with Swiss political and banking stability).
This status isn't permanent or guaranteed — it's a market behaviour pattern built up over time, and it can weaken or shift if the underlying reasons for it change. But while it holds, it means these currencies can strengthen on bad news from somewhere else entirely, which is a frequent source of confusion for anyone expecting a currency to move only on its own country's headlines.