Annotation includes notes scribbled in the margin of an article and highlighted words or phrases. The notes may include questions, comments, words you plan to look up later, and more. Annotating a text is a way to interact with the concepts, arguments, and ideas being discussed. It is an active reading style that helps you to gain a better understanding of a text.
Zotero and other citation management tools includes an annotation option. You must attach a pdf version of the reference to use this tool.