Saber provides liquidity for stablecoins, which are cryptocurrencies whose value is linked to another asset such as the US dollar or bitcoin. Saber is Solana's major cross-chain liquidity network, allowing assets to be transferred between the Solana blockchain and other blockchains. Users put bitcoin into a Saber liquidity pool to earn passive income through transaction fees, token-based incentives, and eventually automated DeFi methods.
Here's where you can learn more about Saber: https://medium.com/cryptobrosresearch/saber-the-leading-cross-chain-stablecoin-exchange-on-solana-47824fb661a1