stETH is a transferable rebasing utility token representing a share of the total ETH staked through the protocol, which consists of user deposits and staking rewards. Because stETH rebases daily, it communicates the position of the share daily.

The mechanism which updates the stETH balances every day is called a “rebase”. Every day at 12PM UTC the amount of stETH in your address will increase with the current APR. You can learn more here: https://blog.lido.fi/steth-the-mechanics-of-steth/
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stETH tokens can be used as one would use ether, allowing you to get staking rewards while using them across the DeFi ecosystem to get more rewards.