The Bitcoin Dominance Index is a percentage that indicates Bitcoin's share of the total market capitalization of the entire cryptocurrency market. When the index rises, it means that money is flowing into Bitcoin at the expense of alternative coins (Altcoins), indicating caution or Bitcoin leading the market. Conversely, when it declines, it suggests an increased risk appetite and a shift of liquidity towards alternative coins, known as "Altcoin Season." In short, it is an important gauge for determining liquidity trends and risk appetite in the market.