Usually: A vacuum leak — a split hose or a perished gasket letting unmetered air in.
But it can also be: A cooling system leak under pressure, which smells sweet, or an A/C component.
Hissing at idle
If your car hisses when it's just sitting there idling, and the sound quiets down or goes away when you rev the engine, the usual cause is a vacuum leak. That means a hose has split, or a gasket has aged and shrunk enough that the engine is pulling in extra air it wasn't built to measure. Engines are tuned on a fairly precise ratio of air to fuel, so this extra unmetered air throws that off.
You can often tell it's a vacuum leak by what else the engine is doing. A vacuum leak tends to travel with a rough idle — the engine sitting a little unevenly, or the RPM slowly climbing and dropping on its own, which mechanics call hunting. Sometimes the check engine light comes on too, because the computer sees the engine running lean and flags it. If you have all three — the hiss, the rough idle, and the light — that's a strong pattern for a vacuum leak.
Two other things can sound similar and are worth ruling out. If the hiss comes with a sweet smell, that points to coolant escaping from a pressurized hose or a small crack, not air. Don't ignore that one — coolant loss can lead to overheating. The other possibility is something in the A/C system letting go, which sometimes hisses too, especially right after you shut the car off or turn the A/C on. If you smell something chemical rather than sweet, or the hiss lines up with the A/C compressor cycling, that's more likely.
As for driving it: a vacuum leak by itself is usually not dangerous. It can hurt fuel economy, make the idle annoying, and over time run the engine leaner than it should for its own good, but it's not something that strands most people overnight. A coolant leak is a different matter — if that's what it turns out to be, keep an eye on the temperature gauge and don't push it if the car starts running hot.
Fixing a vacuum leak generally means finding the specific hose or gasket that's failed and replacing it. The part itself is often cheap. The time is usually in tracing it, because vacuum lines run all over the engine bay and a small split can be tucked somewhere hard to see or reach. We'd want to look at it and probably do a smoke test or similar before saying which hose it is — we don't like guessing on this one, since the wrong guess means paying for a part that wasn't the problem.