I think that part of the problem is that the sentence seems grammatically incorrect. I think that it should be "The explanation FOR this is that different eras call FOR different skills". 1. The first 'for' in the place of the word 'about' is a simple prepositional change. The use of 'about' appears grammatically incorrect to me. I think that I would be more likely to use 'about' to describe a quality of the subject of the sentence. For example, to say something like "the explanation about this is wrong" - but this would not fit properly into the paragraph. Your teacher might be able to help you understand the grammar of these prepositions and how to use them correctly. A native speaker might say something like the original sentence but it doesn't really make sense. 2. The second change is optional. If you were to use 'for' it would relate to a purpose, i.e. different eras REQUIRE different skills. The word 'forth' in the context of 'calls forth' is correct in the original sentence but it has a bit of a different meaning from my version with 'for'. To 'call forth' means to evoke or to create something, in this case different ages (times) create different skills in people. For example, in the past it is likely that people had more practical skills hunting food and fixing things, whereas in today's consumerist age people can just buy everything so those skills are not as important. The skills needed today relate to computers and the large amount of information we have access to, so we now need to be able to process this information quickly. To answer your actual question, as the other people have stated, 'this' is a simple way of referring to the idea you just expressed in the paragraph without having to write the same information out again.

"this" refers to the increased abstract reasoning skills of IQ test takers in the year 2000, in comparison to those who took the test in the year 1950. IQ tests are normally given in public school, so when they say "Today's children..." it is because students are the IQ test takers (normally), and "solving problems on the spot" is a demonstration of that abstract reasoning.

"this" is a pronoun that refers back to the idea in the prior sentence: that kids today are better at solving problems without a previously learned method. It is a short and simple way of referring back to that complicated idea, without having to restate tBe whole idea.