In the sentence ‘It implies a sustained purpose and forbids digressions and irrelevant matter’, what does the word ‘digression’ mean in the given context?

Directions: Read the following passage and answer the question that follows:

Unity is as necessary in a paragraph as in a sentence, and has practically the same meaning for the one as for the other. It means that the paragraph must deal with one subject at a time. It implies a sustained purpose and forbids digressions and irrelevant matter. The unity of a paragraph is or ought to be protected by the theme; for the paragraph ought not to go beyond what the scope of the theme allows. The sentence embodying the theme holds the same kind of relation to the other sentences of the paragraph that the principal clause of a complex sentence holds to the subordinate clauses. As the several clauses of a complex sentence are combined together in construction by conjunctions and relative pronouns, so it frequently happens that the several sentences of which a paragraph is composed are combined together in sense (though they are separated in construction), by words of reference, demonstrative phrases, collateral allusions, and the various other devices of sentence-arrangement.