what does __ double underscores in front of function mean in php code example

Example 1: what does the modularity mean in c++

modularity means the written program can be splitted up in to modules by using classes and each class can be considered as a module.

Example 2: what is monad in javascript

function Identity(value) {
    this.value = value;
}

Identity.prototype.bind = function(transform) {
    return transform(this.value);
};

Identity.prototype.toString = function() {
    return 'Identity(' + this.value + ')';
};