Explorer or Contract Explorer is a tool that allows anyone to easily view and call (permission dependent) all the functions on the smart contract from their connected wallet through the dashboard. This tool also helps developers test function calls on their contract without using a script or a CLI tool.
Write functions modify the state on the blockchain through the contract. Therefore, write functions require gas fees to execute as they are updating the state of the blockchain.
Read functions read state from the blockchain. Read functions do not modify any data, therefore do not require gas fees to execute.