Also known as a 'bachelor party', 'bull's party' or 'buck's party/night'
It's a party or weekend away held for a bachelor shortly before he gets married. A night of drinking and debauchery before he loses his 'freedom'. This may involve he and his party dressing up in weird or outlandish costumes, drinking to excess, gambling, stripper(s) and pranks.
It's a rite of passage from bachelorhood to a supposedly more responsible married life.
The female version of this is called a 'Hen Party'
Dave and 15 of his mates went to Blackpool for his stag party, we dressed him up as a slutty girl, trashed the hotel, got into several fights and ended up spending the night in a police cell.