It is a renewable source of energy, as opposed to fossil fuels. As plants grow they absorb carbon from the atmosphere, using the energy from the sun, in the form of CO2 and return the carbon to the atmosphere by either being eaten by animals and returned as animal biomass or by rotting or burning, again releasing the carbon to the atmosphere.
This is known as the carbon cycle.
By burning fossil fuels we are releasing carbon into the atmosphere which has been locked into the atmosphere many millions of years ago.
By comparison, biomass absorbs carbon from the atmosphere as it grows and releases carbon as it is burnt and if it is managed on a sustainable basis with a continuous replanting programme a closed carbon cycle is maintained and, therefore, becomes carbon neutral.