In AFNI format, the HEAD and BRIK files are pairs of files that are linked. The HEAD files contain a description of the data, but the BRIK files contain the actual volumetric data. The AFNI datasets may be specified with either the HEAD or the BRIK file but not both. So for the example of 3dMean, the example uses only the HEAD files. If a simple * (asterisk) had been used, the shell would have expanded the filenames to include both HEAD and BRIK files.
In the AFNI GUI, for underlay or overlay, you select only the base part of the name, not the suffix (.HEAD or .BRIK) nor the view (+tlrc, +orig, +acpc) for AFNI format datasets.