When you go into ranked queus you get a 'belt' ranking after a few matches which is simply their ranking system.
Quick matches do not offer any belts to you (although they sometimes show incorrectly).
Going from: White<Yellow<Blue<Green<Purple<Red<Brown<Black
The number just shows how close you are to obtaining the next rank or going down a rank, hitting 0 will result to going to a lower tier belt, while hitting 100 results in obtaining the next tier, whilst switching tiers you also start close to 50.
Belts are a meaningless little icon that shows how good a player is. They don't unlock anything or give you powerups. They are just kinda there
Correction: they unlock titles.
Well, that's funny, because I was a Red belt at the end of last season, and I didn't get any titles. Hell, 95% of the people here were Green belt or higher and they didn't get any titles either.