Decomposition involves breaking down a complex problem or system into smaller parts that are more manageable and easier to understand. The smaller parts can then be examined and solved, or designed individually, as they are simpler to work with.
If a problem is not decomposed, it is much harder to solve. Dealing with a complex problem is much more difficult than breaking a problem down into a number of smaller problems and solving each one, one at a time. Smaller problems are easier to understand and can be examined in more detail.
For example, suppose that a crime has been committed. Solving a crime can be a very complex problem as there are many things to consider.
A police officer would need to know the answer to a series of smaller problems:
The complex problem of the committed crime has now been broken down into simpler problems that can be examined individually, in detail. Once the individual information has been gathered and collated, the police officer may be able to solve the crime.
How might you decompose the complex problem of how to create an app?
These smaller problems, solved individually, may help you to create an app.