Every program—like every person—has its quirks, or different ways of doing things. For example, inserting an em dash (as in the above sentence) in WordPress is different from Word.
If you’re used to Word, you know that to insert an em dash in Word, you type two hyphens in a row. The shortcut in WordPress is to type three hyphens in a row. For example, I typed the following into the WordPress word editor:
And it looks like this:
When I want to use an em dash, I type three hyphens in a row—like that.
BTW, the em dash is named because it is the width of the letter “m” in that font.