micronutrients are a subset of "vitamins and minerals" that we only need in small amounts. Your body uses them for physiological processes, but doesn't make them. If you don't get them, the physiological processes they are involved in stop working.

Oxygen is a very oxidizing agent. It used to be a metabolic biproduct which was toxic to most life on earth, until one branch of the tree of life evolved to use it for metabolism. We and most other successful organisms are descended from that branch. Even though we've evolved defenses to counter oxygen's harmful effects, it can still harm us. Using oxygen creates byproducts which are also oxidizing agents. These often go and react with other chemicals in your body, in places they aren't supposed to. This is called "oxidative damage". Anti-oxidants counter this effect - usually by making the oxidizing agent react with the anti-oxidant rather than with important parts of your body. But it's complicated, and you can't necessarily always just isolate a bunch of them and swallow them. That might actually hurt you.