I guess its Soliloquy, but done some research:
The porter's scene is in prose because he is a person of low status. Shakespeare's low status characters nearly always speak in prose to indicate their low station in life. It was assumed these people were of low intelligence, too, and the prose indicates that. The scene is one of comic relief - the still somewhat drunk porter is wakened from his alcohol-induced slumber to answer a persistent knock at the gate of Macbeth's castle. As he stumbles to the gate, he ponders on who might knock at the gates of hell; what sorts of sinners they'd be. There is a unifying theme here - Macbeth has just killed the king and in doing so, lost the ability to say "Amen". He lost all connection with God with this crime. He is doomed to go to hell, so it'll soon be him knocking on hell's gate. The bawdiest lines are 27-35 when the porter tells Macduff the three things that drinking lots of alcohol does to a man.