The following topic is probably the easiest of everything you'll learn in HTML, I've saved it for now because it isn't as useful and commonly used.
Line breaks are horizontal lines that separate different sections of a page.
This will produce the following. Everything is pretty straight forward, just know that size refers to the physical height of the line. Line breaks aren't all that necessary, in fact most web designers don't use them. I also find them handy when I want to place space between two tables. I put that there because I was lazy.