Biochar is a porous form of charcoal that is up to 95% pure carbon, and it results from heating biomass at high temperatures without oxygen. As  a stable form of carbon, it’s an effective end-of-life solution for biomass waste, which would otherwise release CO2 back into the atmosphere when decomposed or burned.
Biochar is one of the carbon removal technologies that are considered medium durability, storing carbon for more than 100 years. This means the carbon is expected to be stored and captured for a similar or greater duration to the life of most of the carbon dioxide in the atmosphere.
The IPCC reports that biochar is one of the safest, most durable, and fastest ways to draw down carbon and capture and store it for long periods of time.
More information on the differences between short-durability solutions, medium-durability solutions, and long-durability solutions can be found here.