Hi there, welcome to the forums!
Without a link to the specific exercise that you’re working on, it’s difficult to be sure, but…
If I remember correctly, the CSS lessons start off with some exercises where you style individual elements by doing something like this:
<p>This is some text and <span style="text-transform: uppercase; font-size: 36px;">this is some text I'd like larger and uppercase</span> compared to the rest of my paragraph</p>
which results in
(I’ve just put that paragraph element into an existing page I had, hence why it’s not the default black text on a white background).
The purpose of the early exercises getting you to style individual elements like this, is precisely to show you how cumbersome (and just how much work it would be) to define and maintain styles in this way.
As you progress, you will move away from inline styles - as I demonstrated above - and towards defining your CSS in a dedicated file and linking it to your HTML document.
You will, as @8-bitgaming has said, continue to rely on
id attributes in your HTML though. These are used to allow you to be precise with which elements get which styles.