stSOL is a liquid stake pool token. This means that you can deposit SOL with LIDO, let them stake the SOL with various validators to earn interest, and gain capital efficiency using the stSOL stake pool token to reprsent your deposit. The risks involved include Lido delegated validators being dishonest and having their SOL slashed when slashing is implemented on mainnet.
How to Mint stSOL
Head to https://solana.lido.fi/ and stake SOL to receive stSOL. Note that it may take 2 to 3 days (1 Solana epoch) to unstake from a validator and receive your SOL back.