A liquid staking token is issued by a staking protocol to depositors as a receipt for their staked assets. The LST (stETH from Lido, rETH from Rocket Pool, etc.) tracks the value of the underlying stake plus accumulated rewards, and can be traded, used as DeFi collateral, or held as a yield-bearing position. The staked asset itself is locked at the validator layer.
LST concentration is a network-level concern. When one LST commands a large share of a chain's total stake, that protocol's governance becomes a chokepoint for the chain's consensus. This is why Lido's market share on Ethereum has been politically charged for years.