What is the difference between field data and lab data in SEO Web Vitals and is the data based on my real visitors?

Field Data is shown for each page at the top level of the report for a quick overview of your metrics. This data is most reflective of the reality of your web page experience as it is an average based on 28-days of actual visits by real users loading your website.

Lab Data is the second data type and is shown when you click the plus icon to analyse a specific page in more detail. All data shown within the Audit Lab Data section is taken from a simulated visit generated by a Google bot visiting your website and measuring each of the metrics for that specific visit. Due to the fact this is based on a single lab visit from a single device and location, the metrics and scores may vary wildly from the experience of your actual visitors and even between individual tests from the same tool.