What is Block explorer?
A block explorer is a search interface for a blockchain. It lets anyone inspect transactions, addresses, and contract source without running a node.
In practice
Explorers such as Etherscan and Solscan are the fastest way to check a claim. You can confirm a contract address matches the project, see whether source code is verified, read the deployment date, and follow where funds moved. Any due diligence on a dApp starts here.