As you build your career, your initial progress is slow. But through deliberate practice, you gain traction, entering into a virtuous cycle that propels you into a sweet spot of accelerating competence and confidence. Then, as you approach mastery, the more habitual the work becomes, the less you enjoy the "feel good" effects of learning: these two cycles constitute the S-curve.
Understanding the S-curve of skill development can help you be less discouraged when you are acquiring new skills, and it also explains why you need to throw yourself a curve once you have achieved mastery of a skill.
Via Bob Corlett