Uphold’s staking feature works with blockchains that use Proof Of Stake (PoS) as a consensus mechanism for validating and processing transactions and creating new blocks in a blockchain.
Owners of a cryptocurrency, pledge their coins to a validator (through Uphold and its partners) as part of this governance process. Once a block is ready to be processed, the cryptocurrency's proof-of-stake protocol selects a validator node to verify whether the transactions are accurate and, if so, add that block to the chain, receiving a reward for their contribution.
The chances of a validator being picked differs with each PoS protocol, with some randomization often employed, but chances are increased by the length of time validators have staked their coins and the amount staked.
Virtually anyone with a minimum balance of a supported PoS token can validate transactions and earn rewards for doing so. Those rewards are credited regularly to your staking account, thereby compounding future rewards. Staking is therefore a great, legitimate way to put your holdings to work for you while supporting the governance function of a blockchain.
Note that rewards earned for staking ETH are not re-staked, but while market conditions allow, Uphold will credit your account as if they were.