What does []() argument in function mean?
That is called a lambda expression and it's a way of including the content of a function as a parameter anonymously instead of writing an actual function and using the name of it.

It's shorthand for:

void myCallback(AsyncWebServerRequest * request) {
    // some code....;
    request-&gt;getParam("Param1")-&gt;value();
}

// ...

server.on("/page", HTTP_GET, myCallback);


It does have a few benefits over traditional functions at the expense of readability, such as the ability to "capture" local variables from the surrounding scope.