Another example: A person wants to buy a company. Everything is going well and looking good. However, the person then discovers that there are problems with the accounts (the financial side) of the company. This would be a setback. The company is now not looking as good as it was before so the person might reconsider buying the company.
Another example: Someone is training very hard to become very good at a sport. Everything is going well, they are improving and the person is very happy with their progress. However, one day they suffer an injury during training. They are told that they must stop training for a month in order for the injury to heal. This is a setback. They have lost a month's training. After that month is over, they will not be as good as they were before the injury so they will have to start training again from a point of slightly lower ability than they were hoping for.
In the article you quote, it appears that the writer is saying that during the school vacation, black students lose more knowledge/ability than the white students. I make no comment on that claim.
At the end of the summer semester, let's say that all the students are at the same level - we'll call it level 1 (the top level). After the school vacation, if the students were tested to see what level they are at, it suggests that the white students are either still at level 1 or perhaps level 2, but the black students are shown to be at level 2 or level 3. They do not seem to retain their level over the summer vacation. I don't think it necessarily suggests that white students engage in different activities over the summer vacation which help them to retain their level. It just suggests that for some reason the black students' level slips down further than the white students' level.
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