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. 

Classifying Good, Needs Improvement, Poor

PSI also classifies field data into 3 buckets, describing experiences deemed good, needs improvement, or poor. PSI sets the following thresholds for good / needs improvement / poor, based on our analysis of the CrUX dataset:

Lab data

PSI uses Lighthouse to analyze the given URL, generating a performance score that estimates the page’s performance on different metrics, including: First Contentful Paint, Largest Contentful Paint, Speed Index, Cumulative Layout Shift, Time to Interactive, and Total Blocking Time.

Each metric is scored and labeled with a icon:

• Good is indicated with a green check mark
• Needs Improvement is indicated with orange informational circle
• Poor is indicated with a red warning triangle

Audits

Lighthouse separates its audits into three sections:

• Opportunities provide suggestions how to improve the page’s performance metrics. Each suggestion in this section estimates how much faster the page will load if the improvement is implemented.
• Diagnostics provide additional information about how a page adheres to best practices for web development.
• Passed Audits indicates the audits that have been passed by the page.

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Source:- https://developers.google.com/speed/docs/insights/v5/about