Learning plateaus are defined as "forward progress apparently stopping while engaged in learning a new skill". It means that you've gotten as far as you're going to get in your learning without doing something drastic or radical to shake things up.
Dutch author Lodewijk van den Broek explained the learning curve in a very simple-to-understand way:
Beginner Stage: You learn very quickly, and the learning curve is steep. (0 to 60%)
Intermediate Stage: You learn more slowly, and it takes longer to progress through this phase. (60 to 80%)
Advanced Stage: This stage is very long, and the learning speed is much slower. (80 to 95%)
Expert Stage: In this stage, you learn at a very slow rate, and the length of the stage is eternal. (95 to 100%) 
So you start out all happy and excited because you're learning something new at a very rapid pace. However, as time passes and you learn more, your pace of learning begins to slow down. You see less and less progress and you get more and more discouraged.
If this is you, you are in a plateau. So how can you overcome your learning plateau?
- Don’t quit -- If you don't keep pushing to learn, you're never going to keep making progress. Sure, it will be slow progress, but some progress is better than none. It's time to force yourself to keep working and learning. Treat your learning like a marathon instead of a sprint.
- Space it out -- It has been proven that you learn better when you space out your learning over time. Don't cram it all into one day per week, but spend an hour or two every day studying. You'll find that you not only retain information a lot better, but you learn more quickly.
- Take risks -- Get out of your comfort zone, and do the uncomfortable thing. Want to learn coding? Start coding your own program or website. Want to learn a language? Travel to a country where that language is spoken and leave the guidebook behind. The more challenges you take on, the more challenges you'll overcome.
- Be deliberate -- When you practice, make sure that your practice is goal-oriented, and that you maintain the proper technique. If you practice with poor technique, that's when you start letting yourself go.
- Refresh yourself on the basics -- Having a hard time learning something new? Going back to the beginning and re-learning the simple concepts may unlock something that you wouldn't have discovered otherwise. Once you have a solid foundation, the more difficult concepts will come in time.
- Be ready to fail -- If you're so worried about failure that you try to do everything you can to avoid it, that's when you're going to make far less progress than you should. Stop worrying about what will happen if you fail, and start working. If you fail, so what? At least you tried, and you've learned what not to do next time. Don't be so afraid of failure that you never try.
Learning plateaus are a part of every learning process, no matter how intelligent you are. It's important to push past and overcome them, it's the only way that you can make progress in life.