I had a real Malcolm Gladwell phase a few years back. In his book, Outliers, he stated that there was an achievement gap between poor and wealth families. However, this gap did not come about during the school term, but rather during the holidays.
This is because the wealthier families did more educational activities with their children than the poorer families during the holidays.
So, this post is stating that some stimulation over the summer is in order. It doesn't have to be the same old stuff you have been doing in the term time. A change is as good as a rest. However, there needs to be some stimulation.
This website summarises the report that inspired the chapter in Gladwell's book and some things to do.