An all in one, or AIO, is a product that aims to compress the kind of work that would normally comprise a several step process into one simple step...so in terms of paint surface care one would usually wash the car first then use an all in one, or AIO that simultaneously cleans the paint by removing some of the gunk that wouldn't simply wash off, beautifies the finish by polishing it, and finally, leaves behind some degree of protection against weathering.
AIO's can be a good place to start if you're just getting into detailing and your car is a daily driver. It's where most of us started, anyhow. As you become more experienced though, you'll find that a multi step process....typically washing, claying, cleaning/polishing, and protecting, and in that order......is really the way to go. The multi step process, though much more time consuming and requiring more skill, almost always leaves the paint looking more radiant as well as instilling a more durable layer of protection.
Oh...and just in case you were curious, there's also a product that's actually called All In One, or AIO. Sometimes you'll see this in posts and it's the specific product to which the poster is actually referring. A little bit confusing...but there it is.
Best of luck getting started in the detailing world.
Chris