When your controller is connected to a console, the player LEDs indicate the controller number.
Also, when attempting to connect to the console or when remaining battery charge is low, the LEDs will flash.
Controller number patterns (horizontal)
When using a Nintendo Switch Pro Controller or Joy-Con held horizontally, the LEDs appear as shown below.
Controller number patterns (vertical)
When using two Joy-Con controllers or when holding the controller vertically, the LEDs appear as shown below.