simpleContract.sol

// This line tells us that the source code is licensed under the GPL version 3.0.
// SPDX-License-Identifier: GPL-3.0

// This line specifies that the source code is written for Solidity compiler version 0.7.0 
// or a newer version of the language up to, but not including version 0.9.0.
pragma solidity >=0.7.0 <0.9.0;

// A contract is declared using a contract keyword followed by the contract's name
contract Storage {

    // This line declares a state variable 
    // called number of type uint (unsigned integer of 256 bits).
    uint256 number;

    // This function store() can be used to modify the value of the variable
    function store(uint256 num) public {
        number = num;
    }

    // This function retrieve() can be used to retrieve the value of the variable.
    function retrieve() public view returns (uint256){
        return number;
    }
}