Refers to a large-scale fire, often one that causes significant damage. Commonly used in news reports.
There was a large fire in the city center last night.
Many houses burned down due to the conflagration.
Last night, a large fire broke out in this district.
During the Edo period, this town was struck by large fires many times.
Many houses were destroyed by the large fire.
大火事 is a more colloquial way of saying the same thing as 大火, which sounds slightly more formal and is favored in written/news contexts.
大火 is a large, devastating fire, the opposite in scale of a 小火, which stays small and is quickly extinguished.
Compound of 大 (large) and 火 (fire). The reading たいか is the on'yomi of both characters.