Almost 20 percent of the Earth's atmosphere is oxygen. Green plants produce it as a byproduct of photosynthesis and it, in turn, is used by most living things on the planet to keep our metabolisms running.
Yet before those photosynthesizing organisms appeared about 2.4 billion years ago, the atmosphere was mostly carbon dioxide, a lot like Mars and Venus.
The common hypothesis is that there must have been a small amount of oxygen in the early atmosphere. Where did this abiotic ("non-life") oxygen come from? Oxygen reacts quite aggressively with other compounds, so it would not persist for long without some continuous source.