There are several common uses for notes in a flow:
Document how a flow works - note cards help to explain the placement or function of one or more cards at a certain point in a flow.
Placeholders for work you intend to do later.
Test values you can use for testing and debugging the flow.
In addition to typing plain text into a note, you can use common markdown to get rich formatting. For a complete list of common markdown options, see http://commonmark.org/help.
Here is a full list of special characters used for markdown that you can precede with a back slash to get the character itself to appear instead of formatting:
< >greater or less than