Each segment is normally one of the twenty amino acids used in Foldit. In some puzzles, one or more segments may represent a "ligand", an atom or small molecule which interacts with the protein. These ligand segments are not "residues".
Segments are numbered starting at 1.
To see more information about a segment, hover over the segment with the mouse and hit the tab key. This opens a "Segment Information" window showing the segment number, the amino acid for the segment, the segment's overall score, and the score parts that make up the segment's score. There may be additional fields shown for some puzzles.