For Heraclitus, fire was the original and most basic element. The element out of which everything was made.
“This world, which is the same for all, no one of gods or men has made; but it was ever, is now, and ever shall be an ever-living Fire, with measures of it kindling, and measures going out.” Die Fragmente der Vorsokratiker by Diels and Kranz (DK), 30
[…] He [Heraclitus] mainly used fire as a metaphor for change, which he thought was the real basis of the universe. Just think of fire for a bit. It never stays the same, it is never calm, it is always in movement. Heraclitus believed that nothing can stay the same for long.
Everything constantly changes and takes the form of other things. The death of one thing is the birth of the other. Fire becomes air, air turns into water, and water into earth:
“Fire lives the death of air, and air lives the death of fire; water lives the death of earth, earth that of water.” DK, 76