CSS stands for Cascading Style Sheets. Where HTML is the code we use to display content on web pages, CSS is how we style that content. When we learned HTML, we learned about inline styles. While those are useful on some occasions, we prefer, in general, to use either internal CSS (a separate section at the top of a web page that defines the style for each item on the page), or external CSS (styling information is stored in a completely separate CSS file and just linked to from our web page). They are called cascading style sheets because multiple sheets can be applied, with inline, then internal and external (whichever one is "last" in the code takes precedence), then whatever the browser default is.
While there is no specific rule about when to use which type (inline, internal, or external), a good place to start is use inline when you just need something quick, use internal when doing just a single web page, and use external when creating a larger collection of pages (a site or part of a site). The power of CSS is that once you decide on a style/format for your page or website, you can then just focus on creating additional content on the page (or pages on the site) - the style will stay consistent throughout the whole page or across the entire site.
To give you an idea of the power of CSS, here is a page created by w3schools.com that shows you the exact same content (defined by the HTML) using 4 different stylesheets (and one with no stylesheet at all).
We're going to learn CSS by using internal CSS, then learn how we can just copy and paste that section of our page to an external stylesheet once we know all the code.