The exprssion is seen in the code snippet below.
For convenience, the question is clearly stated in the comment.

#include <functional>
#include <iostream>
#include <deque>
#include <string>

class Light
{
public:
    Light(std::string name):m_name(name){}

    void on()
    {
        std::cout << m_name << "'s light has been turned on just now" << std::endl;
    }

    void off()
    {
        std::cout << m_name << "'s light has been turned off just now" << std::endl;
    }

    void dim()
    {
        std::cout << m_name << "'s light has been dimmed just now" << std::endl;
    }

    void brighten()
    {
        std::cout << m_name << "'s light has been brightened up just now" << std::endl;
    }

private:
    std::string m_name;
};

class LightInvoker
{
public:
    void add_cmd(std::function<void()> cmd) {
        m_to_run.push_back(std::move(cmd));
    }

    void execute_all_cmd() {
        for (auto& cmd: m_to_run)
            cmd();
    }

private:
    std::deque<std::function<void()>> m_to_run;
};

int main()
{
    auto bedroom_light = Light("bedroom");
    LightInvoker invoker;

    invoker.add_cmd([&]{ bedroom_light.on(); });           //never saw such expression, but it works indeed.
                                                           //why there is a ';' in the {} and there is [&] other than &. I can understand std::bind(&Light::on, bedroom_light);
    invoker.add_cmd(std::bind(&Light::on, bedroom_light)); //I can understand this.
    invoker.add_cmd([&]{ bedroom_light.dim(); });
    invoker.add_cmd([&]{ bedroom_light.dim(); });

    invoker.execute_all_cmd();
}