A rod of ferrocerium that throws sparks at around 3,000°C when scraped with a steel striker. It works soaked, works frozen, works at altitude, and a decent one is good for thousands of strikes.
An honest assessment of where it belongs: this is the third thing you reach for, not the first. Lighting charcoal or a camping stove with a ferro rod is slower and more awkward than a lighter, and anyone telling you otherwise is enjoying the process rather than making dinner.
What it is genuinely good for is being the ignition source that cannot fail in the ways the others do. It does not run out of fuel, it is not affected by cold, and it does not care about being wet. That makes it the sensible third item in a box that already holds a lighter and storm matches.
It needs tinder and it needs practice. Sparks will not light a log or a briquette. They light firelighters, wood wool, cotton wool or birch bark, and those light the fuel. Try it once at home before it matters, because the technique of pulling the striker back against a braced rod rather than scraping forward is the whole skill and it is not obvious.
Buy one with a decent handle. The keyring-sized ones are hard to hold with cold hands, which is when you need it.