What are biofertilizers? In what sense are they better than chemical fertilizers?

Some microorganisms such as nitrogen fixing bacteria, cyanobacteria, fungi and other microflora harbour near the roots of plant inside soil. They convert nitrogen of the air into nitrogenous compounds (ammonia and nitrate) which serve as plant nutrients. Since these microorganisms enrich the soil with nutrients and improve soil fertility, hence also collectively known as biofertilisers.