I want to know what immutable piece of code means in the sense of Ethereum blockchain? Does it mean the smart contract I have developed in the remix IDE cannot be modified or it means something else?
Once deployed, no one can change the smart contract code (including deployer) because of immutable nature of blockchain. In order to change the blockchain, you have to change the whole sequence of blocks because each blocks are mathematically connected to each other like a chain. That's why smart contracts maintains the trust among participants because everyone can view the code before executing any functions (which can't be changed afterwards).