Apart from what others have said, I’ll add another example as a way to think about it: の, the possessive particle.
Similar to possession, “my book” for example, 私の本（わたしのほん）, I am describing the book. Whose book is it? It’s my book. What book is it? It’s my book. The の informs you of the description.
な works in a similar way, and that’s often why when you come across grammar rules, な adjectives and nouns almost always work similarly. (You’re not far enough to have seen that yet, but the pattern will gradually appear!)
It is a book. 本です。ほんです。
It’s my book. 私の本です。わたしのほんです。
It’s an important book. (な adjective) 大切な本です。たいせつなほんです。
The book is important. 本は大切です。ほんはたいせつです。
For the latter sentence, たいせつ is not describing anything specific. As for the book, it is important. Unlike the first sentence, where たいせつ is directly describing the book.
As Kfrl said, the い is the bit of the odd one out, as it doesn’t have any additional attachment to function in either situation. But you’ll see later that it has other forms it takes depending on the situation.
It’s an interesting book. (い adjective) 面白い本です。おもしろいほんです。
The book is interesting. 本は面白いです。ほんはおもしろいです。
Because of this though, い adjectives get to be special, and don’t need a final です・だ to complete the sentence, whereas な adjectives do. (I’m not sure if you’re this far yet, but I wanted to mention it just in case, but no stress if you don’t understand!)
Don’t stress if it takes a while to click. Japanese grammar is the most far removed from English, and a lot of languages really, there is, so it can take some getting used to. Hopefully these posts helped you a bit though. Keep at it!