Etherscan is a website that works for the Ethereum blockchain, which acts like Google's search engine. But only for searching transactions, blocks, smart contracts, Ether addresses, Txn Hashes, Tokens etc. The easiest example is if you withdraw Ether or ERC-20 tokens from any site but there are no transactions in your wallet, then you can copy and paste the Ether address on Etherscan to find out whether the transaction is pending because the blockchain is stuck or traffic is full, or you just need to wait for confirmation.