The self Keyword

A function declared in a class runs on one particular object: the one before the dot in the call. Inside the function, the keyword self refers to that object.

# car.gd
class_name Car
extends RefCounted

var color: String = "Red"
var model: String = "Sedan"

func describe() -> void:
	print("I am a %s %s" % [self.color, self.model])

my_car.describe() runs describe() with self set to my_car, so the function reads that car's variables. You never pass self yourself:

var my_car := Car.new()
var taxi := Car.new()
taxi.color = "Yellow"
my_car.describe() # I am a Red Sedan
taxi.describe() # I am a Yellow Sedan

Writing self. is optional. Inside a class function, color on its own already means the object's color, so print("I am a %s %s" % [color, model]) does the same.

You need self when a parameter has the same name as a variable of the class. Inside the function the name means the parameter, and self. picks the object's variable:

func repaint(color: String) -> void:
	self.color = color

Without self., the line color = color copies the parameter onto itself, and the car keeps its old color.