Explain these lines of code

Hi Friends,
Please help me explain these lines of code?
function startup(&$controller){
$this->controller = & $controller;
$this->set_crumbs();
}
These was written for cakephp 1.1, how to convert them to cakephp 3.6.
Ps: I am very new in cakephp, so i need the help from you.
Thank you very much.