Short Answer
Data are considered as "FAIR" when they comply to the rather general recommendations that all research objects have to fulfil and which were summarized under four major principles Findability, Accessibility, Interoperability and Re-Usability.
Detailed Answer
The FAIR guiding principles for research date were originally drafted in 2015 by the FORCE11 workgroup (http://dx.doi.org/10.1093/biosci/biv095) and later published by Wilkinson et al. (http://dx.doi.org/10.1038/sdata.2016.18). In the meanwhile they are widely accepted and were adopted by several research policies and funding agencies to provide a general guideline how to handle and publish research data (http://dx.doi.org/10.3233/ISU-170824).
They provide not concrete properties, formats or standards that scientific data has to meet, but rather a set of relatively general recommendations that all research objects has to fulfil and which were summarized under four major principles Findability, Accessibility, Interoperability and Re-Usability.
Due to the daily increasing amount of high-voluminous and heterogenous research data the major goal of FAIR principles is to lower the barriers and efforts for humans as well as machines to efficiently discover and reuse research data. This is not only important for the research community who is interested in analyzing the data to get news scientific findings, but also for funding agencies, who provide the budget for producing the data.
All datasets, which were provided via the GFBio portal are aimed to comply the FAIR principles, even if it is currently a bit difficult evaluate this. On promising solution are maybe the lately published FAIR maturity indicators (http://dx.doi.org/10.1038/sdata.2018.118), which provide a suitable approach to measure the FAIRness of datasets and research infrastructures.
!! Link to FAQ-"What is reseach data management"