"False Teaching" can be defined as: when something claims to be, or is taught as representative of -anything- (such as a Faith, or Religion, or set of laws, etc etc.) - but in fact, on deeper investigation and understanding, is not.
In this thread example,
it is popular belief that "being good" (etc. etc.) can earn your way into Heaven/afterlife. This is commonly taught as Biblical, (Even by the Catholic Church), but in fact is utterly contrary to the New Testament. Thus is flatly, False Teaching.