Segmentation is a method of dividing your analytics data into more manageable chunks according to your criteria. You can create a segment, a data subset, of visitors who share certain traits or behaviors. That gives you a broader perspective on your audience and the way they move through different stages of their journey.
You can group visitors based on, for example:
Localization (country, city etc)
Technology (device, browser etc)
Website/App activity, such as:
Specific pages visited
Specific forms filled out
Referral (user comes via search engine, website etc)
That allows you to generate reports for selected visitors according to the chosen criteria.
More about segmentation and segments on the Piwik PRO blog: