Plain meaning
Start with the shortest useful explanation before going deeper.
A predefined fee level applied to a liquidity pool or trading pair, typically expressed in basis points. Different fee tiers exist for different asset volatility profiles: stable pairs (e.g., USDC/USDT) use low fees (1-5 bps), major pairs (SOL/USDC) use moderate fees (5-30 bps), and volatile pairs use higher fees (30-100 bps). LPs select fee tiers based on expected trading volume and impermanent loss risk.