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Here is a sample smart contract for token distribution on the Ethereum blockchain using the Solidity programming language. Then I explain each line.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract TokenDistribution {
    address public owner;
    mapping(address => uint256) public balances;

    event TokensDistributed(address indexed to, uint256 amount);

    constructor() {
        owner = msg.sender;
    }

    modifier onlyOwner() {
        require(msg.sender == owner, "Only the owner can execute this function");
        _;
    }

    function distributeTokens(address[] memory recipients, uint256[] memory amounts) public onlyOwner {
        require(recipients.length == amounts.length, "Recipients and amounts arrays must have the same length");
        for (uint256 i = 0; i < recipients.length; i++) {
            balances[recipients[i]] += amounts[i];
            emit TokensDistributed(recipients[i], amounts[i]);
        }
    }

    function getBalance(address account) public view returns (uint256) {
        return balances[account];
    }
}

1. // SPDX-License-Identifier: MIT
   - This line specifies the license under which the contract is released. Here, it is the MIT license.

2. pragma solidity ^0.8.0;
   - This line specifies the version of the Solidity compiler to be used. It ensures compatibility with Solidity version 0.8.0 or higher.

3. contract TokenDistribution {
   - This line declares a new contract named TokenDistribution.

4. address public owner;
   - This line declares a public variable owner of type address to store the address of the contract owner.

5. mapping(address => uint256) public balances;
   - This line declares a public mapping named balances that maps addresses to their token balances.

6. event TokensDistributed(address indexed to, uint256 amount);
   - This line declares an event named TokensDistributed that logs the distribution of tokens.

7. constructor() {
   - This line declares a constructor function that is executed once when the contract is deployed.

8. owner = msg.sender;
   - This line sets the owner variable to the address that deployed the contract.

9. modifier onlyOwner() {
   - This line declares a modifier named onlyOwner to restrict access to certain functions.

10. require(msg.sender == owner, "Only the owner can execute this function");

    • This line checks if the caller of the function is the owner. If not, it throws an error.

11. _

    • This line is a placeholder for the function body that uses the onlyOwner modifier.

12. function distributeTokens(address[] memory recipients, uint256[] memory amounts) public onlyOwner {

    • This line declares a public function named distributeTokens that takes two arrays as parameters: recipients and amounts.

13. require(recipients.length == amounts.length, "Recipients and amounts arrays must have the same length");

    • This line checks if the lengths of the recipients and amounts arrays are equal. If not, it throws an error.

14. for (uint256 i = 0; i < recipients.length; i++) {

    • This line starts a for loop to iterate over the recipients array.

15. balances[recipients[i]] += amounts[i];

    • This line updates the balance of each recipient by adding the corresponding amount.

16. emit TokensDistributed(recipients[i], amounts[i]);

    • This line emits the TokensDistributed event for each recipient.

17. function getBalance(address account) public view returns (uint256) {

    • This line declares a public view function named getBalance that returns the token balance of a given account.

18. return balances[account];

    • This line returns the balance of the specified account.