It’s a basic concept we are familiar with, tackling tasks one step at a time, only progressing to the next thing once we have mastered the current step. It’s essentially Skill Building 101 and when done properly a solid foundation is built and competency is the ultimate result. Conversely, when a gradient has not been mastered and one tries to move along, confusion ensues.
You would never expect to build a second floor structure if the foundation for the ground floor was incomplete and shaky.
Learning follows the same principle: solid foundation = competency and the basic structure to continue to build upon. 
When you find a student having difficulty in math, take a look at the previous step(s) and find out what they missed and get that addressed and fixed up. Problems with division typically track back to confusions and inability stemming back to multiplication and even subtraction. To focus on handling the division difficulty immediately present is to ignore the earlier source of the real difficulty.
This simple approach applies to any subject and creates the solid building blocks necessary to learning, ability, confidence and competence.
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