Flammable liquids produce vapors which flash or ignite at a given temperature.
When considering how we suppress a flammable liquid fire we still look at
fire behavior basics of the fire tetrahedron.
Regardless of all technical information provided, everything relates back to these basics.

Fuel - the vapors from the flammable liquid

Heat - the original ignition source, ambient temperature, Chemical reactions, flames or other hostile fires.

Oxygen - Already in the air
and the chemical chain reaction.

Fire Behavior Basics  

The above graphic is repeated under "How foam works" but in this section, we are reminded it is based on the fire tetrahedron.

As in any type of suppression, sides of the tetrahedron must be changed such as taking the fuel away or diminishing it, cooling or depleting oxygen by smothering.