a second drink, usualy a shot (ie spirit without mixer) to be consumed with, or after (hence chaser) another drink, probably a pint of beer, or a long mixed dring.
contrast with a mixer, which is a (usualy non alcoholic) drink which is added to spirits to make them more flavourful, or less potent.
When you break the sticks off of bottle rockets, creating a condition where lighting them causes them to fly in all directions, causing widespread panic in a somewhat confined space.