Rock climbing is relatively easy for beginners since the beginner routes are usually not complex, and actually resemble climbing a ladder. As the grades go up, so does the complexity and the difficulty of the routes.
Beginner rock climbers can usually climb a few grades up from the easiest one, even if it's their first time climbing. For example, one grading system for bouldering is v0, v1, v2... As you go up in the number, the climbing gets harder, new holds are introduced that are harder to grip and use, climbing moves become bigger and more difficult, and the route itself becomes more complex.
Many times, beginner rock climbers and even those who it's their first time climbing can climb between v0 and v3. Some can even go up to v4 and v5, though, this depends on the climbing gym, and the climber. If the climber is in good shape and has prior experience in sports that could contribute to climbing, they could very quickly go up to climbing harder routes.
As you go up the grades, the difficulty between each grade also goes up.
If you have ever played a strategy game in which you need to level up, then you can understand how the difficulty works with climbing as you progress.
In strategy games, it is very easy to level up in the beginning, you can usually go up quite a few levels very quickly, but, as you go up in levels, the difficulty between each level increases. By the time you get to what is considered a high level for the game, going up even a single level can take a long time.
The same applies to climbing. In the beginning, new climbers would start climbing v0, which would be considered the easiest grade, but, they would very quickly also be able to climb v1, v2, and even go up to v3 and v4. But, as they go up in the grades, it will be harder and harder to get to the next ones. Beginners may be able to climb some v4s in their first session, but the difference between v8 and v9 is quite huge and will take a lot more time to make the progression, and the difference gets larger and larger as the grades go up.
Think about the fact that the hardest and highest graded route at the time of writing this is a proposed v17. Proposed means that it is not for sure a v17, but the climber who climbed it believes it to be. 17 is not a very high number in general, and not much higher than 10, but the differences between a v10 route and a v17 route are pretty unimaginable for someone who is a v10 climber.
