A customary feature of a literary work, such as the use of a chorus in Greek tragedy, the inclusion of an explicit moral in a fable, or the use of a particular rhyme scheme in a villanelle. Literary conventions are defining features of particular literary genres, such as novel, short story, ballad, sonnet, and play.
The conventions typical in Macbeth are the use of iambic pentameter (see the eNotes link on iambic pentameter ) and the use of imagery (clothing and blood).
While there are other conventions, the one that plays on the themes in Macbeth are central to the repetition of imagery regarding blood and clothing.
The references to clothing not fitting defines the theme of power. Clothing is referenced by the witches during their first meeting with Macbeth. From there out, any reference to clothing speaks to the gain or loss of power that one may have over the play.
As for the repetitive imagery related to blood, this could illustrate the theme of ambition or of fate/free will. Both themes are deeply embedded in the text- they remain central themes throughout the play. (Denoted by another convention of repetition.)