What does ! mean in Solidity?

Question

I'm studying Solidity and I couldn't understand what does [!] mean 

What does it mean? And, how to use it?

Could you give any advice, please?

function sendReward(uint256 _reward, address _dest, uint256 _id) external 

  onlyOwner {

        require(!rewardSent[_id]); // 

        require(_reward > 0); // 

        require(address(this).balance >= _reward); // 

        require(_dest != address(0)); // 

        require(_dest != owner); // 

        rewardSent[_id] = true; // 

        _dest.transfer(_reward);

        emit RewardSent(_dest, _reward, _id);

    }

Answer 1

!= is the inequality operator, so this basically means 'not equal to'.