This is one of those things in the English language that leave you scratching your head. Adding the prefix in- usually turns the meaning of a word on its head: for example, injustice is the opposite of justice. But in the case of the words flammable and inflammable, both mean the same thing — "likely to burn easily and quickly" — and can be used interchangeably when talking about materials, like fuel and cloth. The opposite of these words is non-flammable.
Inflammable can also be used in a figurative sense to mean "easily aroused to anger or passion" (Collins Dictionary).