Chemically, ammonia is a molecule comprising three hydrogen atoms, each linked to one central nitrogen atom. Both very common elements; the Earth's atmosphere is mostly nitrogen, and hydrogen is of course the most abundant element in the universe. That doesn't mean it's simple to produce, but we'll get to that.
At atmospheric pressure, ammonia is a very stinky gas with a boiling point of −33.3 °C (−28.0 °F). Kept cold or under a modest amount of pressure, it's relatively easy to liquefy, making it a much easier green fuel to transport and store than hydrogen. You can truck it about or keep it in tanks, cheap as chips. Hydrogen is nearly 30 times more expensive to store.