There are prospects whose paths to the major leagues are smooth, wide, and uninterrupted, with fresh black asphalt as far as the eye can see. Sometimes it's a longer road, sometimes it's very short, but for prospects whose progression is smooth and simple, we can always track where they are, how they're doing, and assign with a relative degree of certainty what the future holds for them.
For others, however, the road is bumpier, or curvier, or has detours all over the place. In general, of course, when talking about players who have more circuitous routes through the minors, we're often talking about less premium prospects overall.