What is coffee bloom I make drip coffee (pour over for North Americans) at home and I would like to know: what exactly is coffee bloom?

When I pour the initial amount of water the grounds swell up, and sometimes there is a minor eruption if I'm not careful, but is this just air trapped in the grounds and water solution trying to escape or is this CO2?

Does this affect flavour? What affects the volume of the bloom?
The gases themselves are largely composed of CO2 and moisture trapped in the grinds. My understanding is that releasing the gases ahead of time prevent the gasses from interfering with an even extraction throughout the brewing process.

As when brewing coffee, we're trying to expose the grounds evenly to the hot water, the release of gas fights against this, causing channeling or uneven extraction. This can affect your flavor profile by overextracting some of the coffee, and not extracting enough from the rest.

From my own experience, fresher, better-quality coffees tend to produce more gasses. I would imagine this being because the gasses escape on their own over time from the coffee when it's just sitting around.