A fireship was a small vessel, usually of no particular value, that was itself a weapon. It was prepared with combustibles, fitted with a number of small cannon (both as a defence during the setting up of the attack and for increasing the mayhem when then attack occurred) and let loose up-tide or upwind of the enemy in the days of wooden sail-powered warships when fire was the greatest fear of all sailors.
They (there was usually a number of them) were sailed as close as possible to the enemy (normally at night), their helms secured and let drift with a slow match burning or a trail of gunpowder ignited, timed to reach maximum combustion at the time the fireship reached the enemy. Everything about them, their cannon firing, the gunpowder exploding, sending burning stuff far and wide, was calculated to send panic and confusion through the enemy.
Their use has been intermittent, but a famous example was during the battles of the Spanish Armada when the sending of six fireships down on the Spanish fleet moored for the night against the French coast at Calais, forcing them to make an unprepared exit into the North Sea where the English fleet was awaiting them.
The last use of fireships by the Royal Navy was on French ships anchored off the Basque coast of northern Spain in 1811.