LUNA is the native token of Terra blockchain ecosystem. This token plays a critical role in stabilizing the block chain and also locking value in Terra ecosystem. Stabilization of LUNA token is maintained by using LUNA as a collateral and the value is locked in the ecosystem through Staking process.
Any token that is staked in Terra ecosystem is considered as boned LUNA and you can learn more about it over here. The following chart shows how much of LUNA is bonded(staked) vs unbonded in the ecosystem.