what is the black thing or spots on my dog’s tounge

I luv my dog, always i see like when hes tired he like gets his tongue out to breathe like normal dogs right but when i see her tongue I see like a black mark like there’s a black line but not like a perfect line and like the mark is all the way the end of her tongue so I can see her like teeth and others im not like worried i just wanna know what it’s called or does it cause harm or is it harmless of anything like that.