Since the inception of Bitcoin, the amount of BTC owned by the exchanges has steadily increased. However, starting from February 2020, institutional investors have started to withdraw large volumes of BTC from these exchanges for strategic long term holding. Such withdrawals have created pressure on BTC liquidity, and the sell-side crunch has driven up the BTC price since mid-2020 according to the chart below.
Netflow is the difference between exchange inflow and outflow. In the bull market, netflow is likely to keep negative. Otherwise, in the bear market, it's likely to keep positive. For example, in the 30-day moving average, Netflow hit a yearly high at the time of its high in 2018, and Netflow hit a yearly low when Bitcoin was ranging $10,000 in 2020.