What it is
Highlighting is a well-known and often used technique, which probably explains it by itself. What it comes down to is, while reading a text, you mark the portions of the text you think are important (by coloring, underlining, etc.).
How to do it
Use a marker or a pen to color or underline important sections of what you are reading.
Does it work, and why?
This strategy is not useful at all, The only good option is to use highlighting solely as a starting point.
A well-known problem with highlighting is that students think everything is important and wind up coloring their entire textbooks. Students commonly report underlining, highlighting or otherwise marking material. It is simple and quick—but it does little to improve performance. Underlining is ineffective regardless of text length and topic: aerodynamics, ancient Greek schools or Tanzania.
In fact, it may actually hurt performance on some higher-level tasks. One study of education majors found that underlining reduced their ability to draw inferences from a history textbook. It may be that underlining draws attention to individual items rather than connections across items.
In general, students are advised not to use this technique. However, highlighting may be applicable within the context of other strategies. Highlighting or underlining can be useful if it is the beginning of a journey—if the marked information is then turned into flashcards or self-tests. For some students highlighting can help to stay concentrated on the text while reading. However, it has no effect on learning performance at all.