It shows which asset (BTC Vs. stablecoins) is the most used to trade crypto over time. For instance it used to be that ETH/BTC was more traded than ETH/USDT but now the opposite is true. This means that people looking to acquire crypto tends to use fiat rather than Bitcoin. It may seem an obvious choice but back in the day Bitcoin was the only option and even when USDT emerged, many people still favored holding BTC as a reserve asset to acquire other cryptos.