TLDR: Let's say you want to teach an LLM a new skill. You give them training data pertaining to that skill. Currently, researchers believe that this skill development shows up suddenly in a breakthrough fashion. They think so because they measure this skill using some methods. The skill levels remain very low until they unpredictably jump up like crazy. This is the "breakthrough".
BUT, the paper that this article references points at flaws in the methods of measuring skills. This paper suggests that breakthrough behavior doesn't really exist and skill development is actually quite predictable.
Also, uhhh I'm not AI (I see that TLDR bot lurking everywhere, which is what made me specify this).