BNB is a cryptocurrency created by a Binance company. BNB started as an Ethereum-based token (ERC-20), which eventually moved to its blockchain called the Binance Chain. However, unlike Ethereum, Binance Chain has only recently started supporting smart contracts. By the way, you cannot mine BNB as you would a proof-of-work cryptocurrency since the Binance Blockchain uses the Byzantine Fault Tolerance (BFT) consensus mechanism.
Binance limited the BNB offer to 200 million tokens. To counter the depreciation that will occur with the annual reduction in the discount on services. Binance plans to eventually burn half of its total supply, i.e., 100 million tokens. The ultimate goal of "The Burn" is to stabilize the price of BNB over time.