Initializing

Next, add the _ready() function:

func _ready():
    randomize()
    screensize = get_viewport().get_visible_rect().size
    $Player.screensize = screensize
    $Player.hide()

In GDScript, you can use $ to refer to a particular node by name. This allows you to find the size of the screen and assign it to the player's screensize variable. hide() makes the player start out invisible (you'll make them appear when the game actually starts).

In the $ notation, the node name is relative to the node running the script. For example, $Node1/Node2 would refer to a node (Node2) that is the child of Node1, which itself is a child of the currently running script. Godot's autocomplete will suggest node names from the tree as you type. Note that if the node's name contains spaces, you must put quote marks around it, for example, $"My Node".

You must use randomize() if you want your sequence of "random" numbers to be different every time you run the scene. Technically speaking, this selects a random seed for the random number generator.