-> is an object reference.
$this->_scripts[stuff] = "Go to the object $this, and find the _scripts property of it. In this case, that property is an array, and so it calls for a specific index of that array.
$this->params->get('loadMootools') is actually "Go to the object $this, and find the params property of it. This property is apparantly an object itself, with a method get(), which is called with a string parameter (loadMootools)
=> is used as a key-value pairing identifier.