I am a complete beginner in Godot and I try to make a no-tutorial top-down 2D game where the player interacts with food and if he collides with them he gets bigger. The player is a CharacterBody2D and the food is an Area2D. I figured out how to 'destroy' the food entity with a signal, but how is the signal going to notify the CharacterBody2D?

    GameDevWannaBe The point of a signal is to connect it to a function in a node (or multiple nodes) you want to notify. When the signal happens, all the connected functions will be called. You can make connections in the inspector or via the script code. See Using signals section in the docs.

      GameDevWannaBe your character must have an Area2D of it's own to collide on a separate layer, put the item (food) in this layer.
      what you connect is the player's Area2D to the player script.
      when the Area2D collides with something, it will send a signal to the player.
      this signal will cause a function to be executed.

      func _Area2D_collide(body):
     pass

      and will send the parameter body, this is a reference to the thing it just collided with.
      in here you can delete body with queue_free(), or do something else:

      func _Area2D_collide(body):
     body.queue_free()

      because this function belongs to player, you can do changes to player here:

      func _Area2D_collide(body):
     make_player_bigger()
     body.queue_free()

      the point . can be followed by a function or variable, you have to use a function that the node has (look in the docs) or one that you created in a script attached to the node.
      body: Area2D just collided with food, so this is that node food.
      queue_free(): this function is available for every node, it deletes the node.
      body.queue_free(): you are calling the function queue_free() of the node body
      if you don't specify any node, the function will be called for the current script, in this case player.

      for some reason, the brakeys tutorial and many other tutorials teach you to do it the other way around and emit signals from the items. Maybe it's easier for beginners, maybe there's some other reason, but it's not an optimal way to do it.