In daily life, we rarely question mastery. We have mastered knowing our address when we can recite it automatically upon request. We have mastered riding a bicycle or driving when it becomes an automatic skill.
Mastery of academics can be just as clear. We know we have mastered our multiplication facts when it takes very little effort to recall a fact. We know we have mastered reading at a certain grade level when we recognize all of the words on that level easily by sight.
When we teach kids concepts, while skipping prerequisite less-advanced concepts, we do them a disservice. The result can be a graduating senior who has some advanced algebra skills, but still can't count coins, or a middle school student who can read on a high school level, but can't understand what they read because their vocabulary is on a third grade level.
"I would rather have a daughter graduating with a solid seventh grade education" a father said to me once, "than have a daughter with a solid second grade education who had some high-school skills".