Plants need nitrogen to make proteins, but they cannot use the unreactive nitrogen gas in the air. They take in nitrogen through their roots as soluble nitrate and ammonium ions. When a crop is harvested, these ions leave the field with it, so the soil slowly runs short unless a fertiliser is added to replace them.
The Haber process makes ammonia from nitrogen and hydrogen: N2 + 3H2 ⇌ 2NH3. The nitrogen comes from the air and the hydrogen comes from natural gas. The reaction runs at about 450 °C and 200 atmospheres pressure, with an iron catalyst.
The ammonia is then used to make nitrogen fertilisers such as ammonium nitrate and ammonium sulfate. With enough nitrogen, plants make more protein and grow larger, which raises the crop yield. Because of this, the Haber process lets farmers grow far more food from the same land, which helps feed a world population that keeps rising.