Something that is inflammable can be set on fire easily. You dared not light a match after you accidentally spilled gasoline on yourself because you knew you were inflammable.
Inflammable can be a tricky word, since the in- prefix often means "not." But in this case, the in- isn't really a prefix, because the word is built from the verb inflame which means "to set on fire." Some inflammable things might literally go up in flames, but we also use it metaphorically. Someone with a quick temper could be described as "inflammable," and if you fall in love easily, then you have an inflammable heart.