An explorer turns raw chain data into something readable: transfer histories, token balances, contract source, gas paid, and the status of a pending transaction. It is the primary tool for checking what actually happened rather than what an app claims happened.
Use it for concrete tasks: confirming a withdrawal actually left an exchange, checking a token's holder concentration before trading it, reviewing outstanding token-approvals, and reading a contract for an admin-key-risk.
Caution: explorers are third-party services, and their token labels, prices and "verified" badges are not guarantees. Anyone can deploy a token with a famous name. Confirm the contract address from an independent source before you trade.
Related: txid, token-approval, crypto-address