Note taking is the skill of accurately and effectively capturing information from some source(s).
A context where I used this skill:
I use note taking often and in many different formats such as on paper, in text files residing on a computer, and in mind maps. I use this most frequently for trying to figure out how to reproduce difficult to reproduce issues in software. In one role, I was working on a native iPad app that was in its early stages and suffering from memory management issues that would result in crashing software. These crashes would occur occasionally in test but there was no obvious pattern. By recording and later referencing notes, I was able to slowly arrive at a repeatable way to trigger the memory problem and cause the crash to occur.
The notes taken were useful for the process of isolating the behavior which was triggering a memory problem as well as later recording the information in the form of a bug report.
How I'd recommend someone learn this skill:
I developed my note taking skills by taking notes and then assessing their usefulness. This is a sort of self-referencing feedback. If the note are not useful to you, think about why and change your approach slightly next time. There are also specific aspects of note taking ,such as sketching and exploratory testing session notes, that can be developed.