Its a software where you type your maths proofs in and a "yes" comes out. Except if your proof is broken, then a "no" comes out. Of course, sometimes the computer is just a bit dumb at intuiting the intermediate steps, so you need to explain like a 10-year-old child (or else you get a "no" as you failed to convince the computer). And storing all these explanations takes memory. And you have to speak the somewhat idiosyncratic language of the computer, which you can imagine a bit like LaTeX but more easily parseable and less ambiguous.

The blog article author is saying that Lean is like a younger child (that needs more intermediate steps explained), stores proofs more inefficiently (taking more memory) and that its computer proof language is harder to read for humans. Additionally there is a technical point about dependent types, which are the principle around which the computer proof language is organized (the same way a programming language might be organized around object-orientism).