I am familiar with a lot of the reasoning behind some of the more grammatical stuff such as why the possessive of mine can be feminine or masculine depending on the subject but I think that people new to Spanish might appreciate some explanations.
During a lesson, there is a helpful hint link upper right that may have some explanation. Also, if you click on the top line Vocabulary, it will list all the words you have learned, and there are hints and explanations similar to what you may be looking for. I am currently treading water at level 8 trying to learn verbs present, so if you are beyond this point, I am not sure what may be available.
Duolingo is based on a different language learning strategy than you probably encountered in school. The "textbook" method has you working with a lot of grammar in the abstract, and people who are used to that probably expect a lot of in-depth discussion about gender and agreement and declension and so on.
Duolingo's method looks more like the natural way humans learn their first language. No one sits down with a toddler and explains subject-verb agreement in the abstract. (Or if they do, research shows, kids ignore it.) We learn to use the language first, then analyze it later. Some very low-level grammatical insights can speed the process along, but the classic high school hour-long lecture, it turns out, is far less useful than Duolingo's high-input, minimal-analysis approach. (Also, formal grammar is really boring. I'm a linguist, so I'm allowed to say that.)
@briecee: I'm a mathematician, so formal grammar is one of the aspects of language learning which is most interesting to me. I have to force myself to pay enough attention to aspects which are more important for actually communicating in the language, such as usage and vocabulary.^^ You're forgiven for dissing grammar, however. :-)