OSR - Old School Revival or Old School Renaissance
Back in the old days, tabletop fantasy RPGs were a lot simpler and, as a result, a lot faster to learn. OSR refers to current RPG systems using that concept as a basis.
The rules found in 'How to Play' as well as the default traits found in the 'Medieval Fantasy' toolbar emulates this, to a degree.
Some key components are :
> The player character races are human, dwarf, elf, and halfling. Gnome, as an option, is a bonus here.
> The GM controls the world. The players control their characters.
> There are base rules and house rules.
> The characters are heroic, not superheroes.
> Mages are not an unlimited supply of arcane ordinance.
> The basis of the game is not character backstories.
> XP is earned by propelling the game forward with action and adventure, not lolly gagging in taverns, shopping sprees, and trying to score with the hot elven druid/rogue.
There. That's all I feel is needed to explain OSR in Fabletop terms. Much more detail can be found via web search. (Hint - being the Internet, discussions about OSRs vs. 'Story-games' can border on the vile. Just read them for amusement. Don't get involved with the bull%^#}.)