PageSpeed Insights (PSI) reports on the performance of a page on both mobile and desktop devices and provides suggestions on how that page may be improved.
PSI provides both lab and field data about a page. Lab data is useful for debugging performance issues, as it is collected in a controlled environment. However, it may not capture real-world bottlenecks. Field data is useful for capturing true, real-world user experience - but has a more limited set of metrics. See How To Think About Speed Tools for more information on the 2 types of data.