Flammability or inflammability means that something can be set on fire easily. It will burn easily. The words come from Latin. The word at the base is in-flammare (late Latin). It means something like "to put fire to a thing".
Inflammable and flammable are used to mean the same thing. People sometimes get confused and think that inflammable means "not flammable" because the prefix in is often used to mean not. For example, something that is inhuman is not human. Since many people do not know that inflammable means "burns easily", flammable is used more often in public places as a warning.
Flammability can be tested. Paper, for example, is more flammable than wood, and less flammable than natural gas.