What it teaches
Getting information out of a sentence, which is the point of reading and is almost never practised at this length. A child can read every word of a sentence aloud and not be able to say who did what.
One sentence is deliberate. A paragraph brings memory into it, and memory is not what is being tested.
How it works
The sentence appears, then a question about it, then the answers. The sentence stays on the screen while the question is answered: this is comprehension, not recall.
Three levels change the round length, the number of answers, and how much the sentence makes you work.
What to look for
This is the best single predictor on the site of how a child will get on with written work at school. A child who is fast at the other reading games and slow here is decoding well and comprehending less, which is common and worth knowing.