Actually, if you go on the website version of Duolingo, they have a tips section for each "level" and they do explain this there. I wish the same tips were available also on the app but I guess we still have to wait for it (if I remember correctly they have added it on the English course).
A good way to have them at hand is: - Open Duolingo in the browser (I've used Chrome) - Log in - Open the menu of the browser (the 3 dots) - Chose "add to home screen"
I use that when I want to see the grammar tips and go back to the app when I want to do onlu exercises (because it's faster than the website version).
In English, if you say "this is a good book", the grammar helps you emphasize the book. If you say "this book is good", the grammar helps you emphasize the good. Similarly in Hebrew. "This is a good book" (as opposed to that other book) or "this book is good" (and not bad). They are similar but are used slightly differently.