I am just over 50, so still have the ability to interpret from these sub-50 year old adolescents for you geriatrics and vice versa.
Promo / Rel refers to promotion / relegation leagues.
https://en.wikipedia.org/wiki/Promotion_and_relegation
In a nutshell, leagues with multiple tiers where you first must prove yourself at a lower tier prior to advancing to a higher tier in any given season where you can compete for the championship.