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The output of this step is a 4D nifti file (*.nii.gz) with the name as indicated in the -o option, and a _mask file with a similar name. Additional files with the same name as the design matrix but with different extensions are also created: *.con for t-contrast, *.fts for f-contrast.
The 4D file contains values for each subject at the voxels on the boundary of the mean surface (defined by the mask file) as distance from the mean, in voxel units. (Note that the FSL person posted in 2013 that he may change the units to mm at some point.) Here is an example from the hippocampus, where the distance is positive (values can be negative as well).
Analysis options (from UserGuide)
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