Staking derivatives are secondary assets typical of some staking systems in which rewards are not earned with native tokens but through derivative products. In these systems, you receive the corresponding amount of tokens you had locked. For example, if you deposit 32 ETH in a derivative staking protocol, you receive 32 stETH and the staking rewards are paid in stETH. These represent the tokens being staked and the rewards that have been accrued during the staking period.
The value of the staking derivatives depends on the total amount of rewards that are distributed in the network and the performance of the primary underlying asset. They are tradable assets to all effects therefore they can be bought also from someone who does not have the real underlying tokens in staking.