So what is biochar?
Biochar is just charcoal, they slow burn wood by applying heat to it but not allowing to burst into flames. Essentially the wood itself does not burn, but with high enough heat the long chains of carbohydrates it consist of start to degrade, decomposes into several other things (gases, fluids, solids) one of those is char. In the presence of oxygen, the char bursts into flames as it degrades further and falls apart into its components and finally turns into ashes. This is perfect burning, but without oxygen, after a while the whole wood transforms into char.
What makes it "bio"? The effort to sell everything to greenfags overpriced, even stuff that was used by humanity for thousands of years. Everything "bio-" is better. Biokrautchan is the most eco friendly board on the whole chanverse.