<cbgempty> tags are special tags that may sometimes be used to indicate that a paragraph has been left empty on purpose. <cbgempty> tags must be alone in a paragraph, or it won't work.
The most common reason for leaving a paragraph empty is that the source text was incorrectly segmented, broken mid-sentence, in a way that the translator is unable to write anything that would both match the source text and be grammatically correct. They would instead translate the whole sentence in the first paragraph and put <cbgempty> in the second paragraph (and third, etc., in more extreme cases). Incorrectly segmented paragraphs are usually referred to as broken paragraphs.