Once you have staked your SUI, you are contributing to the network's security. In return, you will receive rewards in the form of newly minted SUI. Conversely, if you do not participate in staking, your assets will get diluted over time.
The validator you delegate your tokens to is not able to access your funds at any point in time since you are only delegating your staking rights. At no point in time, we have control over your SUI or your private key.
Currently, you need at least 1 SUI token (+ a bit extra to pay for transaction/gas fees) to stake with.
This website provides you with a comprehensive overview of the ecosystem building upon the Sui blockchain network. Please note that the linked website is maintained by community members of the Sui ecosystem.