Network hashrate is the total computing power of all miners searching for blocks. It is measured in hashes per second (H/s, TH/s, EH/s) and directly affects mining difficulty.
When hashrate grows, difficulty increases and the same ASIC yields less BTC per unit of time. When some miners go offline (e.g. due to electricity prices or bans), difficulty eventually falls. Current hashrate data is available from mempool.space, Blockchain.com and similar services.
In the profitability calculator, network hashrate is used to compute your share: (your hashrate / network hashrate) × block reward × blocks per day.