At the start of The Longing, you get relatively little information. You are a small shade, a sliver of darkness living underground, where the King sleeps. He tells you to wake him in 400 days, and that's when the timer starts.
The game counts down, from 400 days, in real time, giving you an ever-present clock to watch while you wait. Should you wish, you can do nothing, but watch the clock the run for the next year and a bit, standing (or sleeping) by your King. After that time, something will happen (though perhaps unsurprisingly, I don't know what that is).
Alternatively, you can explore. You and the King are sat in a cave network, and with a brave heart and cheerful expression, you can wander out into it, and find things to occupy yourself. The world is simply drawn, with a pleasing cartoonish style. Not overburdened with a lot of sound and music, it gives a strong feeling of loneliness to the shade, wandering about. The occasional thought from your shade interrupts the gloom. After every excursion, however, you will want to wander back home.
This is where the game crosses over into idle-clicker territory. Exploring allows you to find things that can smarten up your small living quarters, such as books, art supplies, nice decorations and a few others. Each small improvement "makes it feel like time is flying by", which is expressed in gameplay terms as a speeding up of the in-game clock. With enough creature comforts, you can bring the waiting down to a fraction of the 400 days, and decide how you should wake the King.
This speeding up of time is helpful, because some of the areas can only be explored after waiting for a certain amount of time. You may need to wait for land to shift, or pools to fill. This can take days, and waiting for it will prove somewhat painful. Luckily, this is a game that counts time even when it's not running, so you can settle down with a book, close the game, and come back tomorrow. And don't forget, when you're wandering around in the darkness, always bring a little light with you.
The game has multiple endings, though I've currently only seen the one, found by a certain amount of exploring. There are hints of other things that can be achieved, but at the minute those hints are a bit nebulous. I'm hoping time will, quite literally, tell. I've had the most fun with it when I'm active, solving minor puzzles and working my way through the cave network, and it's often the tricky challenge of idle games, this balancing of activity against waiting.
Having explored most of the cave (or so it seems at the moment), I'm now settling in for a long wait, though as I say, there may be something else I need to discover. How the game develops towards the end, I can't give you any hints about, but I'll be waiting to see it.
A brief update. I found that secret I was looking for - it would appear there is still more to the game than I had thought - never stop exploring.