Parenthesis is a qualifying or explanatory sentence, clause or word that writers insert into a paragraph or passage. However, if they leave it out, even then grammatically the it does not affect the text that is correct without it. Writers mark them off by round and square brackets or by commas, dashes, little lines and brackets.
i.e. 'Black dog' is the mood of bottomless, suicidal despair suffered, most notoriously, by Winston Churchill (himself a kind of bulldog in nappies, a logo for Empire; growling and dribbling, wheezing smoke, swollen veins fired with brandy)."
(From "Lights Out for the Territory" by Iain Sinclair)