Sepolia and Goerli are test networks. They are primarily used for development on the blockchain (testing out applications and smart contracts, etc).
ETH on these testnets is a dummy balance and cannot be withdrawn or sent to mainnet. This also means that anyone attempting to sell you testnet ETH is trying to scam you: ETH on testnets has no value and is mainly for developers to test out new dapps and features.
See our article on testnet ETH scams for more information.
Changes to available testnets
Ethereum used to have four testnets: Ropsten, Rinkeby, Goerli, and Kovan (Kiln was also added for specific purposes relating to the Merge in 2022). However, after the Merge occurred on September 15th 2022, most of the existing testnets began being deprecated (taken out of service). They may still work, but could be unreliable. Sepolia and Goerli are now the only two available. See here for more information.