Tribal is creature type more or less, a Tribal deck focuses on effects that pertain to and boost that type. (Human, Vampire, Werewolf, Zombie, Elf, Spirit etc...) And humans come in all colors and they have since the beginning of magic so I'm not sure why that is confusing.
Well, yeah, but humans have typically from my experience only been suitable for deck building for white. Sure, other colors had some strong cards that were human type, but not enough rank and file types to build a deck with them, at least when I played. I was never really a competitive player though so... your experience may have varied.