I would be a terrible person to define this, as both these terms see some varied usage. But to my knowledge generally, OSR refers to an “Old School” style of play modeled after OD&D, B/X, and AD&D. The OSR was a movement seeking to return to that style of play, by recollecting and reorganizing the original rules, such as OSE, or Labyrinth Lord or new games based on the sensibilities found in those older games such as Cairn, Knave, or DCC. One of the features that unite these systems is that they are vaguely compatible and that published material can be written in such as way as to not be specific to one system or another and can instead be easily adapted to any similarly styled game. So to write a system-agnostic adventure is to write one with no assumed system, but rather a description of a fictional space that can be easily interpreted and run in any other OSR system. (in this context)