How do unique locks work compared to normal mutex locks?

I've came across with this sample of code provided by a book. Btw this book has bad reviews. I regret that I bought it

std::mutex m_mutex;
mutable std::unique_lock<std::mutex> m_finishedQueryLock{ m_mutex, std::defer_lock };

bool m_playerQuit{ false };
void SetPlayerQuit()
{
    m_finishedQueryLock.lock();
    m_playerQuit = true;
    m_finishedQueryLock.unlock();
}

I'm not satisfied with the book's explanation of how it works and why should I use it. I already have an idea of how mutex locks work and the implementations of it, but I have a difficulty understanding the second line of the code above. And why does it have a mutable keyword in it?

I'm completely new on C++ Programming. So a basic level of explanation would help me a lot. I'm sorry if there are any grammar lapses. English is not my primary language. Thanks.