How are you viewing the page?

If you are viewing the page in Normal View, the lines across the page may just be the automatic page breaks, or even Section Breaks.

For me, I see very little use of the ''Normal'' View. To me, Normal would be the way the page is going to print. That is the view if you change the view to Print Layout View. And Print Layout View is the view I almost always prefer and use. (On occasion I may switch the view to one of the others, but not too often.)

As for getting lines, sort of like fancy underlines, across the page, there are six sets of ''underlines'' that can be set by entering ONLY three of the same repeated characters on the line after the text you want ''underlined''.
(Note: it actually it is a Text bottom border applied to that line. There are a couple dozen different types of borders you can select, but six of them you ''can type'' to have it applied to the bottom of the text line.)

The six characters are Underline (_), Pound (#), Dash (-), Asterisk (*), Tilde (~), and Equals (=).

Try this after typing in some text, press the Enter key to get a new line, than type 3 Pound signs (###), NOTHING ELSE, and then press the Enter key again. You will see the 3 #'s go away, and the previous text will now have a relatively heavy multi-lined underline going across the full text area of the page.

After you see what happens with the 3 #'s, try it again with 3 identical previously named characters. (Hint: I remember those 6 characters from the acronym that spell U P D A T E.)

Anyway, enough fun.
So, if that sort of underline how do you remove it?
Simple, just select the line that has the ''underline" attached to it, and select the command Format>Borders and Shading, Borders, Setting: None then click the [OK] button.