The next year’s schoolbook won’t necessarily present the concept at the same level – the presentation might be too difficult. If a child doesn’t “get it”, they might need very basic instruction for the concept again.
The “how” something works is often called procedural understanding: the child knows how to work long division or knows the procedure for fraction addition. It is often possible to learn the “how” mechanically without understanding why something works. Procedures learned this way are often forgotten very easily.
The relationship between the “how” and the “why” – or between procedures and concepts – is complex. One doesn’t always come totally before the other, and it also varies from child to child. And, conceptual and procedural understanding actually help each other: conceptual knowledge (understanding the “why”) is important for the development of procedural fluency, while fluent procedural knowledge supports the development of further understanding and learning.