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.