Everything I wrote “is Markdown”, because the program you’re using to view my text assumes that my text is formatted in Markdown. You too are writing in Markdown, which for example is how your comment got displayed in bold. You did not “type boldly” to do that, you typed some text like **this is bold** and that got displayed in bold.

Maybe more examples would help. Here’s something I can do because the program you’re using to view my text assumes it’s Markdown: this is a monospace font and this is not. This desire for my text to be displayed in a monospace font is expressed in Markdown using grave quotes. It’s common to use this to denote literal, unprocessed text, so I would say that what I typed was `this is a monospace font`. If you copy and paste that text into a comment, do nothing else to it, and post it, you will see it displayed as this is a monospace font without the quotes because a Markdown compatible program sees it and knows “this person wants the text between these grave quotes displayed monospace”.

You can also see where I just wrote “without” italicized; in Markdown this is expressed as *without* or _without_.

If I type

* Thing 1

* Thing 2

* Thing 3

You’ll see this displayed with bullets, not asterisks, and proper indenting and vertical spacing for a list:

• Thing 1

• Thing 2

• Thing 3

It also gets displayed in exactly the same way if I write it in these two different ways as well:

* Thing 1
* Thing 2
* Thing 3

• Thing 1
• Thing 2
• Thing 3

  * Thing 1
  * Thing 2

  * Thing 3

• Thing 1

• Thing 2

• Thing 3

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Maybe it would be helpful to just skim through a Markdown spec. (There are different flavors of Markdown; this one is called CommonMark, which is usually what people actually mean when they say Markdown. More information on their website.)