Field data means collecting data from the field. In case of pagespeed and performance testing, this means getting data from real users. Unlike lab data from synthetic testing, users won't be visiting webpages under the same conditions. This is why it's important to also monitor user experience of real users, maybe in addition to synthetic testing.
For example, Google's Core Web Vitals is based on field data. Which is good as lab data and field data can be very different, but in the end it's about your very own users/audience. The downside of Core Web Vitals is that data is very generic instead of fine graine-.