The Collateral pool is the pool that keeps the collateralized asset exchanged for stable credit. The value of stable credit issued by the DRS dictates the amount of collateral residing in the pool, which fluctuates in proportion to the exchange rate between the collateral and fiat currency that the minted stable credit is pegged to.
Sometimes when the exchange rates change, the amount of collateral required, and the amount in the Collateral pool diverges. The collateral health check is for checking the actual amount of collateral in the Collateral pool and the required collateral for all the stable credit (issued in the Velo ecosystem) in circulation. It means we can check the state of the Collateral pool and take necessary action to bring the Collateral pool back to balance with rebalancing.