Hypothes.is – a digital annotation tool for collaborative and active reading | Information and digital literacy in education via the digital path | Scoop.it
To define annotation it is when you add notes, comments or opinions about a piece of writing,  or a drawing, photo, or diagram. Often these are critical explanations to add extra insight about something. These explanations can be necessary to understanding writings in which the language might be difficult to make sense of without clarification.