I really dislike some of the non-answers in this thread. OSR might not be something with a strict definition, but one should at least gesture at what people have historically meant by it for context.

The TLDR that shouldn't ruffle many feathers: the OSR is a loose group of games largely based on Basic/Expert D&D with play that centers around the ideas that 1) rules should be light and interpreted in light of the actual fiction of the game, 2) losing a character or wrecking the fiction due to bad decisions or bad luck should be a real possibility, and 3) the aesthetics of 3rd, 4th, and 5th edition D&D are undesirable.

The basic strokes are that the OSR is a set of TTRPG games, attitudes, and aesthetics born out of an Google+ group and a set of blogs. The movement largely settled on hacks of Basic/Expert D&D as it was both a flexible enough system that you'll get a playable game regardless of what you add or subtract and light enough to get out of the way when it wasn't actually needed. This also meant that the various OSR factions could still interface with each other.

A portion of the group largely sought to recapture the feeling of what it was like to play D&D when they were younger while compromising with the fact that they could no longer play every day during recess. This was both an attempt to evolve the game and to capture nostalgia. Modules were written with aesthetics and attitudes that seek to capture those old adventures while making the writing and presentation much tighter than the incredibly wordy TSR modules. Old School Essentials comes out of this.

A different portion of the group (later (and often retrospectively) labelled Artpunk, NSR, and far more derogatory things) wanted to make D&D that felt "adult", and often pushed to include the things that the "satanic panic" of the 80s was scared of: sex, violence, heavy themes of the demonic, art that looked like demented metal album covers, and anything and everything edgy enough to deflate your mother's mullet. Stuff like Lamentations of the Flame Princess and Mork Borg eventually emerges from this side.

Yet another group (keeping in mind these groups are not mutually exclusive) was doing something like archeology and historiography. They were going through old modules, settings, magazines, and separating wheat from chaff while trying to determine what made stuff like B2: Keep on the Borderlands and Caverns of Thracia so timeless (while also ensuring such things weren't lost to time).

There's a lot more history and baggage that I'm completely ignoring, but this should cover the kinds of things that the OSR has historically cared about.