Flammability is a Debuff that provides the chance for Hits to Ignite the target. It is not itself an Ailment, but is closely associated with Ignite, which is.

Fire damage from Hits Contributes to the Magnitude of Flammability, so Hits that deal more Fire damage will Ignite more often.
The base Magnitude of Flammability is 1% chance to Ignite the target for every 5% of the target's Ailment Threshold dealt as Fire damage. Flammability from a Hit is applied before that hit checks whether it should Ignite the target, and so will affect that chance for that Hit to Ignite.

Since chance for Hits to Ignite comes from Flammability on the target, modifiers which increase the chance to inflict Ailments will increase the Magnitude of Flammability inflicted.

Effects which do not Hit targets, but Ignite as if they had, such as Ignited Ground, also inflict Flammability, but do not roll a random chance to Ignite. Such effects will onlyIgnite once the total Flammability on the target reaches 50%.

Multiple instances of Flammability stack, raising the chance for Hits to Ignite the target up to 100%. Each instance of Flammability has its own independent duration, lasting 8 seconds by default.