RETH is the representative token of both how much ETH you deposited and when you deposited these ETH into Rocket Pool ETH protocol, which is a consensus layer (Eth2) beacon chain validator staking protocol.
Rocket Pool is a first of its kind consensus layer (Eth2) Proof of Stake Protocol, designed to be community owned, decentralised, trustless and compatible with staking in Ethereum 2.0. It was first conceived in late 2016 and has since had over 5 successful public betas over the life span of consensus layer (Eth2) development.
Rocket Pool is designed to cater to two main user groups; those that wish to participate in tokenised staking using rETH (opens new window) using as little as 0.01 ETH and those that wish to stake ETH and run a node in the network to help generate a higher ROI than staking outside of the protocol due to commissions earned.