What does it mean that prerequisite relationships create order?

Identifying what must come before what prevents attempting things out of sequence.

Example: In 1962, Robert Gagné published a study of nine-year-olds learning to add integers in columns. He didn't just teach addition. He mapped the prerequisite hierarchy — showing that to add columns of integers, a child must first be able to add two single-digit numbers, which requires understanding the concept of addition as combining sets, which requires being able to identify the numerals, which requires understanding that symbols represent quantities. When Gagné tested children who had skipped intermediate prerequisites, their failure rate was predictable and nearly absolute: 95% of students who lacked an immediate prerequisite failed to learn the target skill. But when all prerequisites were in place, success rates climbed above 90%. The order wasn't a pedagogical preference. It was a structural requirement of the knowledge itself.

Try this: Choose a skill you are currently trying to learn or recently struggled with. Write it at the top of a page. Now work backward: what must you be able to do in order to perform this skill? For each sub-skill, ask the same question — what must come before this? Keep going until you reach things you can already do confidently. You should end up with a tree or chain of 5-15 items. Draw arrows from each prerequisite to what it enables. Circle any prerequisite you have been skipping or assuming you possess but have never explicitly verified. That circled item is your actual starting point — not the skill at the top.