Fractions are introduced early on at a KS1 level, and this is introduced in the form of shapes or measurements of lines, in order to familiarise the child with simple fractions like a half. As the child progresses it becomes more complex, and at a KS2 level they need to know how to add, multiply, divide, and subtract fractions as well as other procedures. Therefore it is important that they have a strong understanding of the basic concept of fractions.
There are many ways to ingrain the basic principle of fractions, one of which being to define what a fraction is in very simple terms until the child grasps the meaning of a fraction i.e. to share. Then from there you can move onto explaining the different parts of the fraction and what they mean etc… but the approach used depends highly on how the child thinks. For example if they are visual thinkers then explaining with shapes and cut-outs would help more than explanations with definitions
Once the basics are ingrained, move onto the harder processes such as the basic operations and mixed fractions etc…