What does the small line on key board mean?

Answer 1

If you mean the key between the zero and the equal, it is hyphen "-" or with the shift key, underline "_".

Hyphen is used in some compound words such as "old-fashioned."

Two hyphens together mean the same as a dash (and some computer programs convert them to a dash). This would be used between phrases, similarly to a semicolon, such as: "Buy now--the sale will end soon!"

The underline can be used to show a blank, such as when the word will be filled in later, such as: "We lived on Green Street from ___________ till November of 2001." On old-fashioned typewriters, to underline a word for emphasis you would type the word, backspace, and add the underline. Now with computers we use control-U to emphasize, like this.