Science, just like Nature, is a multidisciplinary scientific publisher, with a number of journals in different fields, each journal featuring a variety of articles. Apart from articles, letters, and reports (which you have mentioned), they publish reviews and a host of commentary pieces such as book reviews and technical comments/notes. You will find a complete list and description of the various categories of manuscripts (including reports) published by them on this page: Science – Categories of Manuscripts
As per the description on the page, a report presents “important new research results of broad significance.” The best way to understand this better would be to go through a few published reports, which you can find on this page: Science – Reports
Going through the available abstracts of a few recent reports, it becomes clear what the description means. This report around sperm maturation has significance for the broader issue of male infertility, while this one discusses the very relevant challenge of mangrove survival due to a rapid rise in sea levels. It would be best to go through a few reports yourself to understand them better, especially if you are planning on writing and submitting one yourself. In which case, all the best for that!
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