A "line break" is something that ends a line of text, and starts a new line of text. You typically make line breaks by pressing the 'return' or 'enter' key.

However, HTML normally ignores any line breaks in the HTML source of a document; those line breaks do not appear in the web browser. (When we write in HTML, we might be using line breaks to separate our tags without wanting those line breaks to show up in the web browser.)

WikiStyle is simpler than HTML, and line breaks in the WikiStyle source of a wiki page do show up in a web browser. PBwiki does this by converting each WikiStyle line break into HTML <br /> tag in HTML. That <br /> tag tells the web browser to display a line break. For example, if in the Classic editor you write:

Parts of a sandwich:
bread
peanut butter
jelly

You can buy these at the grocery store.

Then PBwiki might use the following HTML:

Parts of a sandwich:<br />
bread<br />
peanut butter<br />
jelly</p><p>
You can buy these at the grocery store.<br />

Because a <br /> tag only makes a line break, there is not anything there to style with CSS. If your stylesheet has a br { ... } declaration, then it will probably have no effect, though with enough work you might find a way to mess up all line breaks on your wiki.

(The </p><p> in the above HTML closes one paragraph and opens another, using the <p> tag. This creates the blank line between "jelly" and "You can buy...".)