Just what it sounds like - there are objects in a picture and you need to spot them. You'd get a list of 10 objects(guitar, owl, hamper, etc) and you need to find them in a picture of a room. The objects frequently aren't hidden per se but are just hard to spit because there might be 100 objects in the picture.
At its core, sure, but then imagine taking that and making it the core mechanic of a larger experience, in the way that "moving between tiles and attacking in a direction" is the core of a roguelike.
You can add time pressure; things that only appear once you've found other things; hybridize the mechanics with those of other genres, like adventure-game-like puzzle minigames to combine or use objects; etc.
In some sense, any game where progression is gated by a threshold of gathered collectables—and where the hard part is detecting the collectable, not navigating to or otherwise retrieving the collectable—is a hidden-object game. Pokemon Snap is a hidden-object game—the objects are shot compositions. The Ace Attorney games are hidden-object games—the objects are literal evidence, but then during the trials they're evidence+opportunity pairings. "Hacking" games, where you detect vulnerabilities to exploit, are hidden-object games. Trade-war or Elite type games (i.e. inefficient-market exploitation simulators) are, on an abstract level, hidden-object games. :)
And, of course, games in other genres can employ hidden-object mechanics, too. Platform games have special coins in out-of-the-way locations, where the challenge is just inferring that that was a visitable place to begin with. RPGs have treasure that shows up as little often-hard-to-spot motes of light on the field rather than big obvious chests. Etc.