Check out example codes for "TypeWriter Text Godot". It will help you in understanding the concepts better.

Code Example 1

extends RichTextLabel

onready var Timer = get_node("/root/Node2D/Timer")

onready var stringPrinted = "This is a test"

func _ready():
    Timer.set_wait_time(.1) # time between letters
    _dialogue( stringPrinted )

func _dialogue( string ):
    for letter in string:
        Timer.start()
        self.add_text( letter )
        yield(Timer, "timeout")

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