1. To add written commentary or notes to text.
Annotation
To add written commentary or notes
to text.

2. What can this look like? You just have to write it down!
It can be the definitions of words.
It can be your own connections; things it made you think of that you might have read, seen or heard before.
It can be your own memories, something it reminds you of.
It can be questions that it makes you think of.
It can be anything that you think about while you are reading.
You just have to write it down!