A term used exclusively in SCA heraldry, it is based on early heraldry, before elaborate arms were developed.
From the Rules for Submission:
Simple armory does not conflict with other simple armory if the type of every primary charge is substantially changed.
These types of changes were normally seen between complete strangers in blood, and were not usually used to indicate any form of cadency. For purposes of this rule, simple armory is defined as armory that has no more than two types of charge directly on the field and has no overall charges.
For example, Or a lion gules only has one clear difference from Or a sword gules. However, because it meets the definition of simple heraldry, and the primary charge has changed, it is considered free of conflict. Note however that even charges that are technically different still may be rejected by the College of Arms if there is not enough visual difference from ten feet: Or a lion gules may be rejected if Or a wyvern gules is also registered.