A preprocessor is any type of system that processes and most times alter data before its being passed onto another system for further processing.
Use Cases and Examples.
Preprocessors are commonly associated with the C and C++ programming language as many modern programming languages do not come with this built-in.
A good example of this is Sass which is a tool that introduces useful CSS syntax to increase developer productivity. It later converts all that syntax down to existing CSS syntax.
There is a very thin line between preprocessors and transpilers though. The former is used to refer to altering data and the latter is used to refer to the conversion of one language to the other.
Here is another article for you 😊 "Letter to an aspiring developer"