Usually: Normal valvetrain noise on many engines, or low oil level on some.
But it can also be: An exhaust manifold leak, which ticks in time with the engine and often fades as things expand and seal.
Ticking at idle
A steady tick while the engine idles is one of those noises that is often nothing to worry about. Plenty of engines make some valvetrain noise when they're cold or just running normally, and it's a sound the owner has probably heard before without ever tracking down. The other common cause is oil level. If the oil is a quart or more low, some engines will tick until it's topped up, because the lifters or the top of the valvetrain aren't getting full pressure.
The first thing to check costs nothing. Pull the dipstick with the engine cold, check the level, and top it off if it's low with the right oil for the car. If the tick was there before topping up and quiets down or disappears after, that's a real answer, not a guess. Write it down anyway, because an engine that keeps dropping oil between changes is worth mentioning to whoever services it next.
The other possibility worth ruling out is an exhaust manifold leak, usually from a small crack or a gasket that's lost its seal. This kind of tick usually keeps time with engine speed and tends to be sharper and more metallic than a soft valvetrain tick. A telling sign: it's often loudest right after startup and fades some as the engine and exhaust warm up and expand enough to close the gap. If you can safely get near the engine bay while it's idling (mind the moving parts and hot surfaces), a tick that seems to come from around the exhaust manifold rather than the top of the engine points this way.
None of this is an emergency. A manifold leak can let exhaust gas escape somewhere it shouldn't, which isn't good to breathe over time, especially if any of it makes its way into the cabin, but it's not something that will strand you or damage the engine suddenly. Low oil is a bit more urgent to address, since running an engine low for a long stretch can wear it faster, but topping it off buys you time to get it looked at properly.
Sorting it out usually starts with a look and a listen: check the oil, check the level history, and check the manifold and gaskets for soot streaks or a leak that's visible or audible up close. If it's oil level alone, that's the fix. If it's a manifold leak, the repair is usually a gasket, and sometimes the manifold itself if it's cracked. Cost varies with the engine and which parts are involved, so it's worth getting eyes on it before assuming either way.