So, the conceit's simple: you corral slimes and collect the gems that, uh, appear when you feed them--and then you can either cash the gems in to buy equipment, or save them to make stuff once you've purchased an extension to your original ranch plot. Although there are things that would like to eat you, there's nothing that can actually kill you--worst case scenario is that you're 'knocked out' for a day in-game.
Along with the ranching aspect, exploration is also a big part of the game, and the world is definitely visually engaging and fun to maneuver through (especially once you get your rocket pack so you can reach some of the higher rock ledges). And with the music turned off, the background ambient noises are a nice nature mix.
I think I watched about 10 minutes of gameplay on YouTube before concluding that I had to play it for myself.
The one caveat I'll offer is that because the slimes function as living animals, there's a fair bit of feeding/cleaning up after them that you have to do on a regular basis. I was talking to a friend about the game, and when he heard about that aspect of it, he immediately declared that he'd have to get a machine to manage all that so that he could focus on the exploring aspect. But that's really missing part of the charm of the whole thing--it's a game about working, but working at your own pace, in a pleasant environment that you can control (once you have enough materials, you can start creating trees and gardens and things around your originally desert-like ranch), with minimally-onerous tasks, and very appreciative "animals" that you're taking care of.
It's really kind of relaxing.
(...And I think I can go squeeze in 20 minutes before I have to go back to the regular grind of my actual job.)