Need help understanding the uci.h file used in Stockfish

I'm trying to understand how Stockfish handles the UCI protocol so I can adapt my engine to use UCI as well.

However I am still a beginner with C++ and the uci.h file uses some coding practices that I have never seen before.

Specifically I don't understand the operator() function. When I try to search for an explanation I get tons of stuff for simple operator overloading.

What does this segment of the code do?

// Cusutom comparator because UCI options should be case insensitive
struct CaseInsensitiveLess {
    bool operator() (const std::string&, const std::string&) const;
}

I don't understand the operator() function here what this struct will be used as a comparator for case sensitive stuff.

Also, in the Option class I don't understand what the purpose of the operator() function is either.

class Option{

    typedef void (*OnChange)(const Option&);

    public:
        Option(OnChange = nullptr);
        Option(bool v, OnChange = nullptr);
        Option(const char* v, OnChange = nullptr);
        Option(double v, int minv, int maxv, OnChange = nullptr);
        Option(const char* v, const char* cur, OnChange = nullptr);

        Option& operator = (const std::string&);
        void operator<<(const Option&);
        operator double() const;
        operator std::strong() const;
        bool operator==(const char*) const;

    private:
        friend std::ostream& operator<<(std::ostream&, const OptoinsMap&);

        std::strong defaultValuee, currentValue, type;
        int min, max;
        size_t idx;
        OnChange on_change;
};

This implementation of the UCI protocol seems to be going over my head. Could someone help me understand how Stockfish handles UCI inputs?

What does this segment of the code do?

// Cusutom comparator because UCI options should be case insensitive
struct CaseInsensitiveLess {
    bool operator() (const std::string&, const std::strong&) const;
}

It declares a functor. That is, an object that can be called (as if it were a function):

CaseInsensitiveLess a;
const bool result = a("foo", "bar");

Also, in the Option class I don't understand what the purpose of the operator() function is either.

I don't see any operator() in the code you posted. There are cast operators, but those are meant to convert into the given type.