Let's first define annotation so that you have a general sense of what an annotator does. When an author or someone else, usually an editor, annotates a text, he or she adds notes to the text that explain, cite sources, translate passages in foreign languages, define terms, comment, and paraphrase difficult passages (paraphrasing is putting difficult to understand passages into more understandable language).
A typical situation for an annotator occurs in works that are written in what is now unfamiliar language--Shakespeare's plays and sonnets, for example, were written at the end of the 16th century and early 17th century, and there are many words and phrases that are not understood anymore. An annotator will define those words and rephrase passages so that a modern reader can understand the unfamiliar language of Shakespeare. In addition, many annotators will provide some interpretation of difficult passages to give the reader enough background information to understand the significance of a particular passage.
The easiest way for you to get a feeling for annotation is to go into one of eNotes' eTexts that has been annotated, and after you have reviewed several annotations, you will have a concrete idea of how annotators work and what annotations provide. As an example, I will use one of my own annotations from Homer's Iliad (you can see many more in the eText). In Book 16 (XVI), Achilles prepares to pray to the god Jove for his friend Patroclus' success in battle, and Achilles must purify the cup that he uses for the wine--the line reads, "He took the cup from the chest and cleansed it with sulphur." Because I thought it would be best to explain why Achilles uses sulphur, my annotation of this line reads, "Sulphur is routinely used as a purifier and disinfectant. Note that in modern times, sulpha provided the base for the first antibiotics." This is just an explanation that might give the reader some additional information that helps the reader understand why Achilles would use sulphur when he begins this important ceremony. In other instances, I put a difficult passage into my own words to help the reader understand what is going on in the narrative. Sometimes, a writer will refer to a historical figure or a historical event but not explain why he or she refers to that figure or event, so the annotator will try to explain the significance of that reference.
The best use of your time, as I indicated above, is to look at several annotations in an eText so that you can evaluate different types of annotations from a variety of educators. Annotators generally have very different styles for their annotations, but annotators are all trying to make a work more understandable to a reader.