An element is a distinct grammatical unit - a 'building block' or segment of a sentence there are three important grammatical elements: word, phrase and clause. Some of the elements of a discourse or text are their sentences, paragraphs, chapters and so on.
English grammar allows certain words to be missed from a grammatical construction (i.e. for a sentence to be grammatically abbreviated) and yet for it still to allow full meaning to be achieved, e.g. 'I bought half a dozen eggs and [...I also bought...] six rashers of bacon.' The reader or listener is able to 'add back in' the elements that have been left out and thus understand what is meant.