A selector is a description of the elements you want, and getting the right ones is the whole skill. Element selectors are broad, class selectors are narrow, and the mistake is writing a selector that reaches more than you meant.
About 1 min of reading
Before this new lesson, one small question about the lesson you just finished. Recall it, do not look it up.
See a selector reach more than you meant.
Broad rule
Before you run it, say out loud what it will print. Then open the answer.
Narrow rule
Before you run it, say out loud what it will print. Then open the answer.
Change one property to something loud, like a red outline, and see which elements change. The visible set is the truth; reasoning about which elements a selector should match is where mistakes hide.
A class called .warning says what the element is for, so the style can change and the meaning stays. A class called .red-text describes the look and forces a rename the moment the design changes.
p { } styles every paragraph; .lead { } styles the ones you marked. The class is the tool for 'these specific ones', and using it from the start keeps a stylesheet predictable as the page grows.
.card h3 { } styles a heading inside a card and nowhere else. That relationship is a normal way to scope a style without adding a class to every element.
h2, h3 { } styles both. Forgetting the comma gives a descendant selector, which is a different rule that usually matches nothing. That one character is the most common selector bug.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.