Why is flattening transparencies important?
It is possible that your document contains images or objects that are partly transparent, which we call transparency. These transparencies are usually not visible in your file, but cannot be processed properly by the printer. This looks like this:
Not flattened
Well flattened
It is therefore important that you flatten files with such transparency. You do this as follows:
Illustrator: choose the option "Flatten transparency" under "Object".
This useful tip is one of the submission conditions.