Since I can't answer to reviews, for the person that didn't understand the rules: they are taken word for word from https://www.puzzle-skyscrapers.com/. If you have a row with (4, 1, 2, 3) or (4, any other numbers), if you were to look at it from the left, you would only see one building, the skyscraper of height 4 blocking the other skyscrapers.
Basically, it represents how many numbers are bigger than the ones before. In a sequence (1, 3, 2, 4), looking from the left, you would see 3 skyscrapers, because 2 is blocked by 3, being behind it, and you only see 1, 3 and 4. You can find a completed level and explanations in the rules section.