1. a device containing gunpowder used for display or in celebration
the fireworks display was spectacular
'Fireworks' are used in colourful displays usually for celebrations such as Halloween, New Year's Eve or special occasions. Fireworks are made from gunpowder and are often colourful.
2. an outburst of anger, trouble
there were fireworks between the men/women
As a colloquial informal expression to say 'there were fireworks' means there was angry shouting between people.