In a text or sequence an n-gram is a sequence of n consecutive tokens. In the case of text treated as a sequence words, a 1-gram is a single word in the text and a 2-gram is a word pair. For example, in this descriotion, "text or sequence a" is a 4-gram. Typically you are interested in the set of all n-grams in a text for some n and their respective frequencies of occurrence in the text.
Used on pages 309, 404